BACKGROUND Intracytoplasmic sperm injection (ICSI) is a more invasive option than conventional in-vitro fertilisation (IVF), which can be successful even when semen characteristics are poor. Reports of higher fertilisation rates after ICSI suggest that this technique may be better than the conventional method for all couples seeking IVF. We undertook a multicentre randomised controlled trial comparing clinical outcome after ICSI or traditional IVF in couples with non-male-factor infertility. METHODS 415 eligible and consenting couples at four UK centres were randomly assigned IVF or ICSI (total 435 treatment cycles: IVF 224; ICSI 211). Usual clinical and laboratory protocols for the two treatment procedures were followed in each of four participating centres. The primary outcome was the implantation rate (number of gestation sacs per embryo replaced expressed as a percentage). Secondary outcomes were pregnancy and fertilisation rates associated with each treatment. Analyses were by intention to treat. FINDINGS The implantation rate was higher in the IVF group than in the ICSI group (95/318 [30%] vs 72/325 [22%]; relative risk 1.35 [95% CI 1.04-1.76]). The pregnancy rate per cycle was also higher after IVF (72 [33%] vs 53 [26%]; 1.17 [0.97-1.35]). Mean associated laboratory time was significantly shorter with IVF than with ICSI (22.9 [SD 12.1] vs 74.0 [38.1] min; 95% CI for difference 45.6-56.6). INTERPRETATION ICSI offers no advantage over IVF in terms of clinical outcome in cases of non-male-factor infertility. Our results support the current practice of reserving ICSI only for severe male-factor problems.