Background and purpose - The best treatment option for severe slipped capital femoral epiphysis (SCFE) is still controversial. We compared clinical and radiographic outcomes of modified Dunn procedure (D) and in situ fixation (S) in severe SCFE. Patients and methods - We retrospectively compared D and S, used for severe stable SCFE (posterior sloping angle (PSA) > 50°) in 29 patients (15 D; 14 S). Propensity analysis and inverse probability of treatment weights (IPTW) to adjust for baseline differences were performed. Patients were followed for 2-7 years. Results - Avascular necrosis (AVN) occurred in 3 patients out of 15, after D, causing conversion to total hip replacement (THR) in 2 cases. In S, 1 hip developed chondrolysis, requiring THR 3 years after surgery. 3 symptomatic femoroacetabular impingements (FAI) occurred after S, requiring corrective osteotomy in 1 hip, and osteochondroplasty in another case. The risk of early re-operation was similar between the groups. The slippage was corrected more accurately and reliably by D. The Nonarthritic Hip Score was similar between groups, after adjusting for preoperative and postoperative variables. Interpretation - Although D was superior to S in restoring the proximal femoral anatomy, without increasing the risk of early re-operation, some concern remains regarding the potential risk of AVN in group D.