Purpose Predictive factors for long-term outcome after in situ fixation for chronic slipped capital femoral epiphysis (SCFE) have so far not been systematically analysed. The aims of this study were to define predictors for long-term outcome and to assess the association between initial grade of slipping and post-slip deformities. Methods We studied 51 patients (60 hips) with chronic SCFE treated with in situ fixation. There were 31 males and 20 females with a mean age of 12.9 years. The mean follow-up was 39.0 years (21.1 to 56.8). Post-slip deformity was defined as abnormal alpha (alpha) angles in anteroposterior (AP) and lateral radiographs; AP angle of 74 degrees and lateral angle of 63 degrees were used as cutoffs between normal and abnormal alpha-angles. Harris hip score (HHS) less than 85 points was classified as a poor clinical outcome and osteoarthritis was a poor radiographic outcome. Results The mean slip angle was 31.7 degrees (12 degrees to 77 degrees The mean AP alpha-angle was 67.4 degrees (36 degrees to 111 degrees and the mean lateral a-angle was 57.1 degrees (22 degrees to 104 degrees Post-slip deformities developed in almost one-third of hips with a mild degree of slipping and in about half the hips with more pronounced degrees of slipping. The long-term outcome was good in 40 hips (67%) and poor in 20 hips. A small AP alpha-angle was the only independent prognostic factor for a good combined clinical and radiographic outcome. Conclusion Post-slip deformity, defined as an abnormally high AP alpha-angle, is the most important prognostic factor for poor long-term clinical and radiographic outcome in chronic SCFE treated with in situ fixation.