['In situ fixation (ISF) is standard treatment for slipped capital femoral epiphysis (SCFE) to stabilize the epiphysis and to prevent further slip. The aim of this study was to clarify the incidence of slip progression after ISF and its prognostic factors.', 'We retrospectively reviewed 53 hips in 49 consecutive SCFE patients who underwent single screw ISF and were followed until physeal closure. Clinical and radiographic findings were viewed to assess progression of the posterior tilting angle (PTA).', 'Mean PTA was 33.4 degrees (range, 18 to 75 degrees) at ISF and 35.9 degrees (range, 18 to 75 degrees) at physeal closure with progression of PTA of 2.5 degrees (range, -2 to 19 degrees). Slip progression occurred in 28 of 53 hips (53%), and more than five degrees of progression occurred in 14 hips (26%). Multiple regression analysis revealed that point of screw insertion (one point for lateral and two points for medial) was a significant prognostic factor for progression of the slip by the following formula: (progression of PTA)\u2009=\u2009-1.523 + 2.701 × (point of screw insertion), R(2) = 0.148, p = 0.005.'] Is lateral insertion a good prognostic factor after in situ fixation in slipped capital femoral epiphysis?