A total of 111 women with no ultrasonographic findings of polycystic ovarian syndrome were observed between January 1989 and December 1991 in an in-vitro fertilization (IVF) programme. The treatment schedule involved ovulation induction after treatment with a gonadotrophin-releasing hormone (GnRH) agonist, using standard doses of human menopausal gonadotrophin (HMG) for 4 days, and further stepwise increments in dosage as required. Response to the treatment was defined as: (i) presence/absence of one or more follicles > or = 10 mm diameter after 4 days of treatment, and (ii) oocyte retrieval. Three indices of body mass were considered: weight (W) in kg/height (H)2 in metres (Quetelet's index), W/H1.5 (the National Health and Nutrition Examination Survey anthropometric index for women), W0.30/H (ponderal index). Surface area was computed as 0.0235 (H in cm0.422) (W in kg0.515). Women in the upper tertiles of the range of anthropometric indicators more frequently tended to present no follicle > or = 10 mm on day 7. Likewise, oocytes were retrieved less frequently in subjects in the higher tertile than in those in the lower. The odds ratio of a negative response both on day 7 and at the end of treatment increased with the tertiles of body mass indices or surface areas. This study suggests that response to ovulation induction treatment is inversely related to body mass index.