In 10-20% of patients undergoing total hip replacement bilateral replacement is necessary. This is usually done in two stages. The aim of our study was to investigate the influence of the interoperative period in bilateral hip replacement. We performed a retrospective study of 96 patients in whom bilateral hip replacement had been performed between 1974 and 1984; 72 of these patients were followed up. Assessment on follow up was based on a mobility scale, the survival time of the first prosthesis implanted was elicited from all patients. Consideration of mobility of the patients and survival time of the first prosthesis implanted revealed that the best results were obtained with an interoperative interval of between 1 and 3 years. When the interval had been shorter the mobility was less good, while patients with an interoperative period of more than 3 years had progressively less good mobility and shorter survival times of the first prosthesis. Although the range of indications for total hip replacement is not influenced by these results, the indications for replacement of the contralateral hip must be reconsidered in view of the importance of the interoperative interval.