Women with anovulation due to polycystic ovary syndrome are likely to develop multiple follicles during gonadotrophin therapy and therefore have a high risk of multiple pregnancy. We have developed a low-dose regimen for use in these women; 100 women with clomiphene-resistant polycystic ovary syndrome were treated. Ninety-five of the women ovulated at least once, 72% of the 401 cycles induced were ovulatory and the majority (73%) of these were uni-ovulatory. The overall cumulative conception rate was 55% at 6 months with only two multiple pregnancies. The rate of early pregnancy loss was 32%, which is similar to that reported by other groups. The prevalence of complications was low with no cases of severe hyperstimulation and less than 5% of cycles were abandoned because of development of multiple follicles. Analysis of baseline and mid-follicular luteinizing hormone levels showed that a raised baseline and/or mid-follicular luteinizing hormone level was associated with a poor response to treatment, i.e. anovulation, ovulation but no conception, or early pregnancy loss. There were no successful pregnancies in the women whose luteinizing hormone levels were persistently raised during ovulatory cycles. Low-dose gonadotrophin therapy is a safe and effective method of inducing ovulation; it is associated with a high incidence of single follicular development and a very low multiple pregnancy rate.