The optimal time period for intrauterine insemination (IUI) in relation to either luteinizing hormone (LH) surge or human chorionic gonadotrophin (HCG) administration leading to the best pregnancy rates has not been determined. In this study, 856 consecutive human menopausal gonadotrophin (HMG)-stimulated and 49 natural unstimulated IUI cycles carried out at a reproductive medicine unit affiliated with a tertiary centre were analysed in a retrospective fashion. There were three scenarios in the temporal relationship of the LH surge, HCG administration and artificial insemination. These were (group A) subjects who had an endogenous LH surge but were not given HCG; (group B) subjects who were given HCG after an observed LH surge, and (group C) subjects who were given HCG before the LH surge. The overall pregnancy rate (PR) was 16% per cycle. The PR was 9% in group A, 20% in group B and 14% in group C. The PR in group B was significantly better than group C (P = 0.04). In group B, the longer the time interval between the LH surge and HCG administration, the better the PR up to 20 h (P = 0.025); the timing of IUI based on the LH surge was not critical to the achievement of pregnancy within 3 days. In group C, PR improved with the increasing interval between HCG and IUI from <28 h up to 60 h. We conclude that a better PR is achieved if a spontaneous LH surge occurs before HCG administration, especially where the administration of HCG is delayed 8-20 h after an observed LH surge; the timing of IUI based on the LH surge is not critical to the achievement of pregnancy within 3 days.