An 11-year-old female spayed domestic shorthair cat was referred to the Veterinary Hospital Gregorio VII, Rome, Italy, because of a history of meloxicam-responsive haematuria of 2 months’ duration complicated by the acute onset of pollakiuria and stranguria after an unsuccessful attempt at cystocentesis. On abdominal palpation a firm, non-painful mass was palpated in the caudal abdomen. Haematology and biochemistry showed increased aspartate aminotransferase (63 UI/l; reference interval [RI] 0–40 UI/l), alanine aminotransferase (81 UI/l; RI 0–50 UI/l), urea (81 mg/dl; RI 20–60 mg/dl) and creatinine (1.91 mg/dl; RI 0.35–1.50 mg/dl). Urine specific gravity was 1.035 (RI 1.001–1.065), but a complete urinalysis was not processed because of our inability to collect an adequate urine sample. Right lateral abdominal radiograph projection showed an over-distended urinary bladder with irregular margins (); retrograde contrast urethrocystogram was suggestive of a urinary neoplasia (, ). The cat was scheduled for abdominal ultrasound the following day. On abdominal ultrasound, the urinary bladder was over-distended by the presence of an oval, vascularised mass of mixed echogenicity measuring 3.5 × 5.2 cm (); the lesion raised from the craniodorsal aspect of the urinary bladder wall and the trigone appeared not to be involved. Signs of chronic bilateral kidney disease, kidney mineralisation, a reduced-sized left kidney (2.8 cm) and bilateral pyelectasia (1 mm) were also detected. Ultrasonographic findings were consistent with degenerative kidney disease and urinary bladder neoplasia. Thoracic radiographs were unremarkable. The owner declined CT and opted for explorative laparotomy. The cat was anaesthetised and a ventral midline coeliotomy was performed. The urinary bladder appeared over-distended and firm, with an irregular dorsal wall on palpation. The urinary bladder was filled by a large mass of fibroelastic consistency (). Stay sutures were placed in the apex of the bladder and paramedian along the ventral aspect to the level of the bladder neck and proximal urethra. The urinary bladder was isolated from the remainder of the abdominal cavity with laparotomy sponges. A ventral cystotomy was performed to examine the bladder mucosa and the mass was found to be infiltrating the craniodorsal wall of the urinary bladder and extending into the lumen. The mass was close to the ureter openings; therefore, after urethra catheterisation () a partial cystectomy with gross debulking was performed. After debulking, there was visible residual tumour ventral to the trigone. All vital parameters remained stable during anaesthesia. On gross examination, the mass was white to pink-reddish in colour, irregular in shape and smooth but not encapsulated; the mass measured 5 cm × 4.2 cm × 3 cm, and few scattered ulcerations and haemorrhages were noted on the surface. The mass was routinely fixed in buffered 10% formalin and stained with haematoxylin and eosin. Histologically the mass was composed of a proliferation of spindle cells with a nucleus to cytoplasm ratio of 2:1; cytoplasm was scant, basophilic and frequently bipolar. The nuclei were pleomorphic, ovoidal and basophilic; one or two nucleoli were evident. The cells were loosely and irregularly disposed in bundles with interstitial basophilic matrix interposed (); new diffuse vascularisation with red blood cell engorgement and focal haemorrhages were also seen. The mitotic index at 10 high-power fields was <5. Histological examination was consistent with a malignant, moderately differentiated mesenchymal tumour suggestive of a fibrosarcoma. Immunohistochemistry was performed and was strongly positive for vimentin (Vimentin Ab-2, 1:100 in antibody diluent; Thermo Fisher Scientific) () and negative for S-100 protein (Mouse S-100 Ab-1; Thermo Fisher Scientific), Desmin (Desmin Ab-2; Thermo Fisher Scientific) and smooth muscle actin (Perossidasi DAB detection; Dako), confirming the diagnosis of fibrosarcoma. The cat was hospitalised on supportive care, fluid take over, meloxicam (0.05 mg/kg q24h SC, Metacam; Boehringer Ingelheim), amoxicillin/clavulanic acid (20 mg/kg q12h IV, Synulox; Pfizer) and methadone hydrochloride (0.2 mg/kg q6h IM, Metadone Cloridrato; Molteni). A 5 Fr Foley (SurgiVet; Smiths Medical) catheter was placed. The cat exhibited severe gross haematuria over the first 48 h of hospitalisation with subsequent severe anaemia (packed cell volume [PCV] 14%). At the same time, on repeated blood work, an increased azotaemia was noted (). Worsening of azotaemia was considered secondary to acute kidney injury due to hypovolaemia and hypotension secondary to urinary blood loss. After blood typing and cross-matching, the cat received two packed red blood cell transfusions. Haematuria and azotaemia improved over the following days. The cat was discharged 10 days after surgery with a PCV of 26% and mild azotaemia (). Abdominal ultrasound re-examination at time of discharge was characterised by the presence of a 1.86 cm × 1.30 cm inhomogeneous mass arising from the dorsal aspect of the urinary bladder wall (). Given the ongoing chronic kidney disease, low-dose metronomic chemotherapy with cyclophosphamide was proposed but declined by the owner. The cat was regularly re-checked after discharge. On subsequent physical examination, the owner reported the absence of any clinical signs; however, ultrasound re-examination at 4 months after surgery detected a small increase in the size of the residual mass. Eight months after surgery, the cat was euthanased because of acute onset of gross haematuria followed by stranguria; biochemistry and ultrasound examination repeated by the referring veterinarian were consistent with severe azotaemia and progression of the tumour that was causing bilateral ureteral obstruction and hydronephrosis.