In a 14-year-old girl with a 5 month history of left sided thigh pain, X-rays demonstrating osteoarthritis and a bone scintigraphy showing increased focal uptake in the femoral neck led to the diagnosis of an osteoid osteoma. The protuberant bone at the femoral neck was removed en-bloc and the diagnosis histologically confirmed. The synovium was extensively infiltrated with lymphocytes. Postoperatively the girl experienced neither the expected pain relief nor improvement of her hip function over the next 4 months. MRI and CT results indicated development of a severe osteoarthritis even though no residual lesion could be found.