Background and purpose — The value of arthroscopic surgical reduction in developmental hip dysplasia is poorly known. We compared the clinical and radiographic efficacy of arthroscopic and medial open surgical reduction in patients less than 18 months of age with developmental hip dysplasia.Patients and methods — 54 patients with a mean age of 11 months who were treated by Ludloff’s medial open reduction technique (28 hips, Group L) or arthroscopic surgical reduction technique (26 hips, Group A) were evaluated in this case series. Data on age, sex, preoperative Tönnis grade, operative time, estimated blood loss, residual leg length discrepancy, range of motion (ROM), acetabular index (AI) angle, coverage ratio of the femoral head, continuity of Menard–Shenton line, re-dislocation rate, McKay classification, and Kalamchi–MacEwen avascular necrosis (AVN) classification were collected.Results — Preoperatively, the mean AI angle was 39° in Group L and 37° in Group A. At the latest follow-up, the mean AI was 26° in both groups. The mean femoral head coverage ratio was 79% in Group L and 80% in Group A. The Menard–Shenton line was intact in all patients. Residual leg length discrepancy or limited ROM was not detected in any patients. 4 patients in Group L and 2 in Group A were diagnosed with type 2 AVN.Interpretation — Arthroscopic surgical reduction in patients aged 6–18 months revealed promising clinical and radiographic outcomes similar to medial open reduction using Ludloff’s technique.