On the basis of intraoperative observations in 13 consecutive adolescents (14 hips) with slipped capital femoral epiphysis (SCFE), we found that when the anterior femoral metaphysis was level with or extended past the epiphysis, it caused labrum and cartilage damage. As a result of an impingement between the metaphysis and the superomedial acetabular rim, the labrum revealed erosions, scars or tears. Further jamming of the metaphysis into the joint damaged the adjacent acetabular cartilage, varying from a partial- to a full-thickness cartilage loss. In all patients, the femoral head cartilage was intact; no avascular necrosis was present. Our findings suggest that arthrosis in SCFE can be triggered by early mechanical damage of the acetabular cartilage.