The radiolucent cartilage space of eighty-three patients with unilateral or bilateral slipped capital femoral epiphysis was measured by a standardized technique. In the majority of patients, whether unilateral or bilateral involvement, there was bilateral narrowing of the cartilage space. In the unaffected hip of unilaterally involved patients, there was a progressive narrowing as skeletal maturity was attained. A concomitant anatomical study of cadaver hips, removed at autopsy from adolescent patients, showed a progressive narrowing of the cartilage as the proximal femur matured. Black females showed most narrowing (minimum cartilage space width), had the narrowest final cartilage space widths, and took the longest to attain this final width. While other racial, sexual and therapeutic groups failed to demonstrate statistically significant differences, the general trend was for females, Blacks, and patients treated by osteotomy to have more joint space narrowing. However, rewidening occurred in most of these affected joint spaces, in contrast to the progressive linear decrease observed in unaffected hips and anatomical specimens. On the basis of this study, we feel that cartilage space narrowing may be anticipated in the post-operative period in most patients treated for slipped capital femoral epiphysis. This narrowing appears to improve with time. Narrowing of greater than one-half the original width, in association with pain and limitation of joint function, probably represents "cartilage necrosis," or pathologic joint space narrowing. Unless the narrowing remains less than one-half to two-thirds of the initial cartilage space for more than twenty-four to thirty-six months, probably no specific surgical treatment should be undertaken, other than observation and protected weight bearing during any painful phase. Plotting the roentgenographic cartilage space width during the three month to thirty-six month phase may be useful in monitoring and predicting the outcome.