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The presence of femoroacetabular impingement (FAI) after a slipped capital femoral epiphysis is thought to predispose the subsequent development of osteoarthritis (OA); however, there is a lack of evidence to support this hypothesis.One hundred twenty-one patients with stable slipped capital femoral epiphysis treated with in situ fixation were reviewed at a minimum of 20-year follow-up; the presence of a pistol grip deformity and FAI was determined. The Harris Hip Score (HHS) was used to measure clinical outcome, and the Tönnis grade for qualifying the presence of OA was determined.One hundred twenty-one patients were followed up at a mean of 22.3 years (range, 20.1 to 32.5 y); the slip was considered grade 1 in 34 hips, grade 2 in 65 hips, and grade 3 in 22 hips. Ninety-six patients had clinical and radiographic signs of FAI. The mean HHS for the entire cohort was 75.6; however, for the 25 patients without FAI it was 89.3 and for the 96 patients with FAI it was 75.4 (P=0.004). We found radiographic signs of OA in all 121 patients: considered grade 1 in 14 hips, grade 2 in 32 hips, and grade 3 in 75 hips. The mean Tönnis grade of OA was 2.5. A direct relationship between the radiographic grade of OA and the HHS was observed.The occurrence of FAI (or a pistol grip deformity) after even a low-grade slip is common. We found clinical and radiographic signs of FAI in most of our patients, and also found that the degree of deformity is directly related to the presence of OA in early adulthood.