We have tried to establish the course of an isolated osteochondral defect in a femoral condyle would take if it were not treated surgically. We studied 15 knees in 12 patients where a diagnosis of the condition had been made in the past and no surgical treatment instituted for it. Each patient was examined clinically and had an MRI scan. The average period of follow-up was 109 months, the range being 54-282 months. Six patients (seven knees) were under the age of 18 at the onset of symptoms. Of these, MRI of six knees showed either no abnormality or a healed lesion. Where the onset was after skeletal maturity, six of the eight knees showed features osteoarthritis on the MRI scan. Symptomatically, there was also a difference between the two groups.