A 47-year-old male admitted to our department with pain and swelling on his right calf. The patient was diagnosed with RA 5 years ago. He had no history of knee trauma. On physical examination, tenderness of wrists and elbows, swelling on the left wrist and contracture of right elbow were found. McMurray test was negative bilaterally. There was no swelling in the left knee. Right knee flexion was limited and Ballotman test was positive. There was bilaterally knee joint tenderness with palpation. He had painful swelling and redness on his right calf. There was a persistent itching on the skin of the right calf. Homans test was positive on the right. Plain radiographs showed mild degenerative changes at knee joints. Erythrocyte sedimentation rate was 22 mm/hour, C-reactive protein was 24.7 mg/L, WBC count was 14350, rheumatoid factor was positive. Complete blood count except for WBC and biochemical laboratory tests were within normal limits. These findings on the calf were similar to deep vein thrombosis. We used ultrasonography for differential diagnosis of deep vein thrombosis. Ultrasonography revealed massive fluid collection within the fascial compartments and gastrocnemius muscle layers. There were two septas between fluid collections. Color Doppler ultrasonography showed normal flow pattern in popliteal veins. According to these findings, the patient was diagnosed as Pseudothrombophlebitis syndrome due to rupture of Baker’s cyst. 280 cc inflammatory fluid was aspirated from three different points under the guidance of ultrasonography. After the intervention, the swelling on the right calf decreased. The patient was relieved and elastic bandage was applicated to his calf. After 1 week follow-up, the swelling on his calf repeated and another intervention was performed. Approximately 100 cc inflammatory fluid was aspirated and 1 cc steroid (betamethasone) was injected into right knee joint. After 1 month follow-up, we found that the pain and redness on his calf was clearly decreased. The patient’s pain visual analog scale (VAS) value was decreased from 8 to 2. We observed that the patient continued to improve at follow-up visit 3 months later; VAS value was 0 and there was no swelling or redness on his calf.