Developmental dysplasia of the hip (DDH) is a very common congenital disorder, and late-presenting cases often require surgical treatment. Surgical reduction of the hip may be complicated by avascular necrosis (AVN), which occurs as a result of interruption to the femoral head blood supply during treatment and can result in long-term problems. Some surgeons delay surgical treatment until the ossific nucleus (ON) has developed, whereas others believe that the earlier the reduction is performed, the better the result. Currently there is no definitive evidence to support either strategy.To determine, in children aged 12 weeks to 13 months, whether or not delayed surgical treatment of a congenitally dislocated hip reduces the incidence of AVN at 5 years of age. The main clinical outcome measures were incidence of AVN and the need for a secondary surgical procedure during 5 years' follow-up. In addition, to perform (1) a qualitative evaluation of the adopted strategy and (2) a health economic analysis based on NHS and societal costs.Phase III, unmasked, randomised controlled trial with qualitative and health economics analyses. Participants were randomised 1 : 1 to undergo either early or delayed surgery.Paediatric orthopaedic surgical centres in the UK.Children aged 12 weeks to 13 months with DDH, either newly diagnosed or following failed splintage, and who required surgery. We had a target recruitment of 636 children.Surgical reduction of the hip performed as per the timing allocated at randomisation.Primary outcome - incidence of AVN at 5 years of age (according to the Kalamchi and MacEwen classification). Secondary outcomes - need for secondary surgery, presence or absence of the ON at the time of primary treatment, quality of life for the main carer and child, and a health economics and qualitative analysis.The trial closed early after reaching < 5% of the recruitment target. Fourteen patients were randomised to early treatment and 15 to delayed treatment. Implementation of rescue strategies did not improve recruitment. No primary outcome data were collected, and no meaningful conclusions could be made from the small number of non-qualitative secondary outcome data. The qualitative work generated rich data around three key themes: (1) access to, and experiences of, primary and secondary care; (2) the impact of surgery on family life; and (3) participants' experiences of being in the trial.Overoptimistic estimates of numbers of eligible patients seen at recruiting centres during the planning of the trial, as well as an overestimation of the recruitment rate, may have also contributed to unrealistic expectations on achievable patient numbers.There may be scope for investigation using routinely available data.Hip 'Op has highlighted the importance of accurate advance information on numbers of available eligible patients, as well as support from all participating investigators when conducting surgical research. Despite substantial consultation with parents of children in the planning stage, the level of non-participation experienced during recruitment was much higher than anticipated. The qualitative work has emphasised the need for appropriate advice and robust support for parents regarding the 'real-life' aspects of managing children with DDH.Current Controlled Trials ISRCTN76958754.This project was funded by the National Institute for Health Research (NIHR) Health Technology Assessment programme and will be published in full in ; Vol. 21, No. 63. See the NIHR Journals Library website for further project information.