OBJECTIVE To determine the relative value of the woman's age, basal follicle stimulating hormone (FSH), basal luteinising hormone (LH) and menstrual cycle pattern (all defined prior to treatment) in predicting the ovarian response to gonadotrophin stimulation for in vitro fertilisation. METHODS Open, descriptive cohort study. METHODS One hundred and seventy-one consecutive ovulatory women undergoing their first cycle of in vitro fertilisation with ovarian stimulation after pituitary desensitisation. METHODS Measurement of basal (early follicular phase) and down-regulated concentrations of serum FSH and LH, and calculation of age at treatment and of median length of menstrual cycles and range of variation prior to treatment. METHODS Duration and amount of gonadotrophin stimulation to achieve follicular maturity, number of mature follicles, peak serum oestradiol concentration and number of mature oocytes, were compared between banded values of variables studied. RESULTS Increasing basal FSH concentrations were significantly associated with reducing oestradiol levels, numbers of mature follicles and oocyte yield even when the FSH levels were still within the normal range though above average (6 to 8.9 i.u./l). There were similar but weaker associations with FSH levels after down-regulation. There were also significant differences between women over and under 40 years old in their oestradiol levels, numbers of follicles and of oocytes. Analysis of variance showed significant independent association of basal FSH with both oestradiol and numbers of oocytes, but not of age, menstrual pattern, or serum LH concentrations. CONCLUSIONS Serum FSH is more sensitive than the woman's age in determining her ovarian responsiveness to stimulation. LH concentrations and menstrual cycle patterns are unhelpful.