SummaryTwenty-three core biopsy specimens from patients with slipped capital femoral epiphysis and 11 control growth plates were studied by electron microscopy. Block staining with toluidine blue for demonstration of proteoglycans at the ultrastructural level was also used. The proliferative and hypertrophic zones of slipped growth plates showed diminished cellularity and marked distortion of the architecture with disarray of the cartilage columns. There was a deficiency and abnormality in the supporting collagenous framework of slipped plates. Chondrocyte degeneration and death were seen at all levels of the proliferative and hypertrophic zones, suggesting a disturbance in the life cycle of chondrocytes. The significance of these findings and their relationship to mechanistic, endocrine, and other proposed etiologies of epiphyseal slipping is discussed.