We aimed to determine the effectiveness of the vaginally administered spermicide nonoxynol-9 (N-9) among women for the prevention of HIV and other sexually transmitted infections (STIs). We did a systematic review of randomised controlled trials. Nine such trials including 5096 women, predominantly sex workers, comparing N-9 with placebo or no treatment, were included. Primary outcomes were new HIV infection, new episodes of various STIs, and genital lesions. Five trials included HIV and nine included STI outcomes, and all but one (2% of the data) contributed to the meta-analysis. Overall, relative risks of HIV infection (1-12, 95% confidence interval 0.88-1-42), gonorrhoea (0-91, 0-67-1-24), chlamydia (0-88, 0-77-1-01), cervical infection (1-01, 0-84-1-22), trichomoniasis (0.84, 0-69-1-02), bacterial vaginosis (0-88, 0-74-1-04) and candidiasis (0-97, 0-84-1-12) were not significantly different in the N-9 and placebo or no treatment groups. Genital lesions were more common in the N-9 group (1-18, 1-02-1-36). Our review has found no statistically significant reduction in risk of HIV and STIs, and the confidence intervals indicate that any protection that may exist is likely to be very small. There is some evidence of harm through genital lesions. N-9 cannot be recommended for HIV and STI prevention.