The incidence of osteochondritis dissecans in the condyles of the femur was studied in a defined population and was found to be twice as common in men as in women; the maximum incidence in both sexes being between the ages of 10 and 20. The site of the lesion in the condyles did not differ between age groups. The indicence of diagnosied cases has increased somewhat in recent years but only in men. The incidence in the population is less than has been suggested in the past.