Objective: To evaluate the effectiveness of 15 minutes of immobilisation versus immediate mobilisation after intrauterine insemination. Design: Randomised controlled trial. Setting: One academic teaching hospital and six non-academic teaching hospitals. Participants: Women having intrauterine insemination for unexplained, cervical factor, or male subfertility. Interventions: 15 minutes of immobilisation or immediate mobilisation after insemination. Main outcome measure: Ongoing pregnancy per couple. Results: 391 couples were randomised; 199 couples were allocated to 15 minutes of immobilisation after intrauterine insemination, and 192 couples were allocated to immediate mobilisation (control). The ongoing pregnancy rate per couple was significantly higher in the immobilisation group than in the control group: 27% (n=54) versus 18% (34); relative risk 1.5, 95% confidence interval 1.1 to 2.2 (crude difference in ongoing pregnancy rates: 9.4%, 1.2% to 17%). Live birth rates were 27% (53) in the immobilisation group and 17% (32) in the control group: relative risk 1.6, 1.1 to 2.4 (crude difference for live birth rates: 10%, 1.8% to 18%). In the immobilisation group, the ongoing pregnancy rates in the first, second, and third treatment cycles were 10%, 10%, and 7%. The corresponding rates in the mobilisation group were 7%, 5%, and 5%. Conclusion: In treatment with intrauterine insemination, 15 minutes' immobilisation after insemination is an effective modification. Immobilisation for 15 minutes should be offered to all women treated with intrauterine insemination. Trial registration: Current Controlled Trials ISRCTN53294431.