A 10-year-old boy who was otherwise healthy visited a private practice due to high fever, which had persisted for about 1 month. A blood test showed a high level of C-reactive protein (13.1 mg/dl; normal value: < 0.01 mg/dl), so he was referred to a nearby hospital for a further investigation. Abdominal ultrasonography at that hospital revealed a solid large tumor in the liver (approximately 6.3 × 5.3 cm at the time). Enhanced computed tomography (CT) showed that the tumor was mainly located at S4 and S5 in the liver. The tumor was well-circumscribed and showed gradual weak enhancement from the arterial phase to the portal phase. On magnetic resonance imaging (MRI), the tumor showed low intensity on T1-weighted imaging and a high intensity on T2-weighted imaging, with partial diffusion restriction. 18F-fluorodeoxyglucose-positron emission tomography (FDG-PET) showed the marked uptake of FDG by the tumor, with an early maximum standardized uptake value (SUVmax) of 8.8. There was no evidence of distant metastasis on any of the imaging modalities. The levels of tumor markers, including α-fetoprotein (AFP), protein induced by vitamin-K antagonist-II (PIVKA-II), carcinoembryonic antigen (CEA), and carbohydrate antigen 19-9 (CA19-9), were all within the normal limits. Based on these findings, the most likely differential diagnosis was suspected to be undifferentiated sarcoma; thus, he was referred to our hospital for surgical treatment. First, to confirm the diagnosis and develop an operation plan, a core needle biopsy was performed at our hospital. Based on the findings of the preoperative examination, including the biopsy results, the preoperative diagnosis was suspected hepatocellular carcinoma. The Child–Pugh score was 8 (albumin, 3; prothrombin time, 2; bilirubin, 1; ascites, 1; and encephalopathy, 1), and the indocyanine green retention rate (ICG-R15) was 2.9%. On enhanced CT just before the operation, the tumor was 8.7 × 10.4 × 13.1 cm in size, showing marked growth within a one-month period. We planned to perform elective surgical resection by laparotomy. At the operation, a large mass was located in the central portion of liver with no findings of peritoneal dissemination or intra-abdominal metastasis. The tumor involved liver segments 4, 5 and 8; however, the dorsal portion of segment 8 had been spared. The tumor did not actually invade Glisson’s sheath at the liver hilum, and the distance between the tumor and the umbilical portion of portal vein was 1 cm. After removing the gallbladder, hepatic parenchymal transection was performed with a water-jet hybrid knife (erbe JET2®; Erbe Elektromedizin GmbH, Tubingen, Germany) using the Pringle maneuver while confirming the tumor location using intraoperative ultrasonography. The dorsal portion of Segment 8 was successfully preserved. The blood supply of the area was confirmed by intraoperative ultrasonography after resection. Finally, the tumor was resected en bloc with a margin. The operative time was 521 min, and the blood loss was 490 ml. He was transferred to the previous hospital on postoperative day 12. Macroscopic findings showed a yellowish white, solid tumor of 10 × 9 cm in size with hemorrhage and necrosis. Although the tumor was well-circumscribed macroscopically, in a histopathological examination, the tumor cells showed an infiltrative growth pattern and vascular invasion was observed. The tumor was composed of polygonal or oval-shaped cells arranged around blood vessels, spindle cells arranged in fascicles, and round epithelioid cells with clear cytoplasm. Tumor cells showed a high nuclear grade and multinucleated giant cells were also noted. Mitotic figures were easily recognized; the mitotic activity was 30/50 hpf, including abnormal mitosis. Immunohistochemistry revealed that some cells were positive for α-SMA and melan A, while approximately 50% of cells were positive for HMB-45. These histopathological findings, along with immunoreactivity with melanocytic markers, were consistent with a diagnosis of perivascular epithelioid cell tumor. The diagnosis was also confirmed by a central review committee in the Japan Children’s Cancer Group (JCCG). After discharge from our hospital, he was followed up at another hospital. In the 6th month after the initial surgery, he complained of shoulder pain. MRI showed dumbbell-like shaped tumor at the 2nd thoracic vertebrae, which was confirmed to be bone metastasis of PEComa by biopsy. After reducing the tumor size by chemotherapy (including ifosfamide and doxorubicin), vertebrectomy was performed. Postoperatively he suffered from cerebrospinal fluid leakage and meningitis, which were treated by vancomycin. Additional therapy was judged to be unnecessary because no viable cells were found in the specimen. At two years from the relapse, in a regular follow-up visit, thoracoabdominal CT showed a 10-cm solid mass occupying the pelvis and a 15-mm nodule in the middle lobe of the right lung. The pelvic tumor was extirpated by laparotomy, while a nodule in right lung was removed in a thoracoscopic procedure. A pathological examination revealed that both lesions were PEComa, and genetic alteration of the TSC2 gene was identified in tumor cells. At four months after second relapse, pelvic metastasis appeared again. Since the third relapse, he has been carefully treated with a mammalian target of rapamycin (mTOR) inhibitor.