A prospective clinical and laboratory study was performed in thirty-four patients (twenty-two blacks, ten Puerto Ricans, and two whites) who had slipped capital femoral epiphysis in fifty-two hips. Although the majority of the laboratory studies were normal, all patients showed significant elevations of the serum immunoglobulins and C3 component of complement, with the highest values recorded for IgA. Urinary glycosaminoglycans were elevated in the few patients studied and increased proportionally with the duration of the disease. Nine (25 per cent) of the thirty-four patients had chondrolysis in thirteen hips. The male:female ratio in these nine patients was 0.8:1, compared with the ratio of 1.4:1 in all thirty-four patients. No additional biochemical abnormality was found in the patients with chondrolysis except for a greater elevation of the IgM fraction. These preliminary and tentative data suggest either that slipping of an epiphysis produces an antigen which induces an autoimmune state or that slipping is a localized manifestation of a generalized process resembling some form of connective-tissue disorder or inflammatory state. There is presumably a genetically determined sub-group of patients with this disorder who may have chondrolysis.