Although external fixation methods have been described for proximal femoral osteotomy for various etiologies, none are dedicated to a single disease entity. Our study introduces a technique of proximal femoral osteotomy and fixation with a monolateral external fixator system in Legg-Calvé-Perthes disease (LCPD). Twenty-three patients (19 males, four females) with LCPD underwent surgery at our institute between 2004 and 2012. Varus osteotomy (group A, 11 hips) and valgus osteotomy (group B, 12 hips) were performed and the monolateral external fixator system was used. The average age of patients at surgery was 13 years (6–23 years) and the mean follow-up duration was 21 months (12–64 months). The mean angular correction of the varus osteotomy in group A was 20° (10°–28°) and the mean medial displacement was 21% (10–49%). The angular correction of valgus osteotomy in group B was 28° (14°–49°) and lateral displacement was 41% (38–58%). The mean fixation time was 14 weeks (8.4–31 weeks). Complications occurred in nine hips (39.1%) and included pin-tract infections (five), hip abduction contracture (one), nonunions (two), and refracture (one). Our surgical technique provides precise correction and stable fixation with minimal intervention. Therefore, the monolateral external fixator could be considered an acceptable alternative fixation device for the correction of proximal femoral deformities in patients with LCPD. Level of evidence: Level IV, case series.