We report a rare case of pseudothrombophlebitis resulting from spontaneous rupture of a Baker's cyst in a 54-year-old man with ankylosing spondylitis. The presentation of a painful, swollen calf is clinically difficult to differentiate from thrombophlebitis. The diagnosis was confirmed by multi-detector computed tomography which showed a characteristic crescent fluid accumulation behind the calf muscles. An accompanying soft tissue infection was also found. The patient was successfully treated with bed rest, elevation of the leg and antibiotics Prompt correct diagnosis of this rare condition is crucial. This entity should be included in the differential diagnosis of unilateral painful calf swelling.