PURPOSE OF THE STUDY When developmental dysplasia of the hip (DDH) is irreducible by conservative means, then surgical open hip reduction is the choice. When done before walking age simple open hip reduction (SOHR) is most often enough to stabilize the hip. We tested the hypothesis that simple open hip reduction gives enough stability even in severe Tönnis 4 grades of dislocation. We tried to find what is the cut-off age to safely perform SOHR with lower complications. MATERIAL AND METHODS From 193 open hip reductions (OHR) of irreducible DDH in 123 children for the period 1995-2010 year with X-ray follow up of at least 8 years and full documentation we investigated 75 SOHR. Mean age at follow-up was 13 years and 5 months for the whole group and 8 years and 3 months for the SOHR patients. Age at operation - 7 months to 7.5 years for all patients with OHR, with average OR age for SOHR - 18 months. The traditional surgical technique of open hip reduction was performed through lateral Murphy's approach but with sparing the attachment of the piriformis muscle with the underlying blood supply to the epiphysis during circumferential capsulotomy. We used McKay clinical criteria, radiological classifications of Tönnis, Severin, Herring-Mose and Kalamchi & MacEwen. RESULTS When analyzing the results, there came up a strong statistical correlation between bilaterality and Tönnis grade 4. The final CE angle of Wiberg was on average 28º. According to Severin classification: 74% were excellent and 10% good or 84 % successful results. But according to Herring-Mose sphericity scoring - 60% were good and 30% fair. Mose's fair is a potential cam-type femoro-acetabular impingement (FAI). Clinically according to McKay criteria - 10% excellent and 54% good, or 64% of the operated hips were clinically acceptable, which correlates more with the radiological results according to Mose than with Severin. Avascular necrosis (AVN) of the femoral head (FH) according to Kalamchi was: Type I - 6%, Type II - 12 %, Type III - 0%, Type IV - 8 %. Types II and IV represent 20 % of the operated hips. The presence of ossific nucleus in the femoral epiphysis on initial X-ray didn't have a statistical impact on AVN frequency. When analyzing the impact of severity of dislocation and trying to find the cut-off age for fewer complications, there came up a strong statistical significance between AVN IV type appearance in Tonnis grade 4 hips when age at operation was above 9 months. CONCLUSIONS The thorough capsuloplasty after FH reduction in the acetabular socket is enough for stability even in Tonnis 4. When OR age is below 9 months severe AVN is lower and Herring-Mose FH sphericity scoring is higher. Key words: DDH, simple open hip reduction, avascular necrosis of femoral head.