Background. Habitual hip subluxation and dislocation is a potentially disabling feature of Trisomy 21. We describe long-term outcomes following precise use of the femoral varus derotation osteotomy to achieve and maintain hip stability and community ambulation. Methods. All individuals with Trisomy 21, who had hip surgery at Toronto's Hospital for Sick Children between 1998 and 2008, were searched using the hospital databases. 16 hips in 9 children aged less than 10 years, were identified. All had a femoral varus derotation osteotomy (VDRO) with a target femoral neck shaft angle (NSA) of 105° and less than 20° external rotation. All were performed by the senior author JHW. The clinical notes and radiographs were reviewed from presentation to final follow up. Continuous variables were assessed for normality with the d'Agostino Pearson test. Normally distributed variables are presented as means with 95% confidence intervals. Pre and postoperative means were compared using the student's t-test for paired samples. Results. Mean age at first known hip dislocation was 4.56 years (3.98, 5.15), mean age at surgery 6.07 years (5.15, 7.0) and mean follow-up 5.4 years (3.75, 7.06). Mean femoral neck shaft angle (NSA) fell from 166.7° (162.2°, 171.1°) to 106.0° (103.1°, 110.2°). In 2 hips, significant intraoperative instability persisted and periacetabular osteotomies with capsulorraphies were required. All patients developed an asymptomatic waddling gait postoperatively. 14 hips developed peritrochanteric varus deformities, mean 20.7° (15.96°, 25.4°)2 (12.5%) sustained periprosthetic fractures 4 and 8 years postoperatively. 1 hip (6.2%) developed painful arthritis. None re-dislocated at latest follow up. Conclusion. Prevention of long term disabling pain from habitually dislocating or recurrently subluxed hips, in patients with Trisomy 21, requires surgical intervention to increase hip containment. We believe surgical management of habitually subluxed or dislocated Trisomy 21 hips allows for a predictable result. We recommend a varus producing proximal femoral osteotomy to correct the NSA to 105° with less than 20° of external rotation. This technique was successful at restoring and maintaining hip stability in 16 hips with a mean follow-up of 5.4 years, whilst maintaining the patient's ability to remain community ambulators. It should be performed before aged 8 or signs of a widened or V-shape teardrop develops. In our experience this has been effective in maintaining hip stability. However, little is known about the natural history of habitual hip dislocation in these children. Patients with Trisomy 21 are living longer with hips that are at continuous risk. Thought should be given to the potential benefits of periodic screening of this treatable hip condition