A 48-year-old woman had a 2-week history of increasingly severe right posterior ankle pain after she began a walking program 4 weeks earlier. The results of plain radiography were reported as normal. Achilles tendonitis was suspected, with the possibility of an early calcaneal stress fracture. Bone scintigraphy revealed changes consistent with severe tibialis posterior tenosynovitis. Because of the intensity of changes in the medial malleolus, an infective cause was considered possible. Surgical exploration and biopsy revealed a severe fibrinous tenosynovitis and associated periostitis of the medial malleolus. Culture of biopsy material grew no organisms. Because no other cause could be identified, the inflammatory response was ascribed to overuse injury. The patient was treated with a corticosteroid injection into the tendon sheath, with good effect.