BACKGROUND: Prophylactic fixation of the contralateral hip in cases of slipped upper femoral epiphysis is controversial. Therefore, using a single-cannulated screw has been widely accepted. However, differing reports exist on the occurrence of persisting growth after prophylactic epiphysiodesis. The aim of this retrospective study was to evaluate the presence of persisting growth of the upper femoral epiphysis after prophylactic fixation. METHODS: From 2006 until 2009, 11 children underwent prophylactic pinning using a single-cannulated 6.5-mm cancellous screw. Time to fusion, persisting growth, and overgrowing of the screw were measured on plain radiographs taken postoperatively and at least after the growth plate was fused. RESULTS: All patients except 1 (91%) showed a persisting growth of the epiphysis, and in 2 cases a hardware replacement was necessary. The mean increase of the femoral neck length was 8.2% (SEM 1.46%). Mean follow-up was 37 months (range, 12 to 49 mo). All patients had a Risser sign grade 0 at the time of surgery, and equal or less than grade 3, when the growth plate was fused. CONCLUSIONS: Despite previous reports that a prophylactic fixation using a single-cannulated cancellous screw is unproblematic and safe, we showed that in our series growth persistence was the rule and in some cases the physeal overgrowth necessitates a hardware replacement. Careful follow-up until fusion of the growth plate should be recommended.