The present study investigates the usefulness of inhibin A, inhibin B and serum oestradiol concentrations obtained in the fifth day of gonadotrophin therapy in predicting ovarian response and assisted reproductive treatment outcome in women undergoing ovarian stimulation under pituitary desensitization. A total of 80 women undergoing their first cycle of in-vitro fertilization (IVF)/intracytoplasmic sperm injection (ICSI) treatment were studied. Twenty consecutive cycles which were cancelled because of a poor follicular response were initially selected. As a control group, 60 women were randomly selected from our assisted reproductive treatment programme matching by race, age, body mass index, and indication for IVF/ICSI to those in the cancelled group. For each cancelled cycle, three IVF/ICSI women who met the matching criteria were included. Basal follicle stimulating hormone (FSH) concentrations were significantly higher in the cancelled than in the control group, whereas basal inhibin B was significantly higher in the latter. Basal oestradiol concentrations were similar in both groups of patients. On day 5 of gonadotrophin therapy serum concentrations of oestradiol, inhibin A and inhibin B were significantly lower in the cancelled group as compared with controls. Logistic regression analysis showed that the association for day 5 inhibin B (with a predictive value of ovarian response of 91.03%) with cancellation rate was significant, independent of, and stronger than, the effects of any other hormone variable investigated. In addition, day 5 inhibin B concentrations were correlated directly with parameters of ovarian response, ovum retrieval and oocyte and fertilization outcome. However, day 5 inhibin B was not a better predictor of pregnancy than the other hormone variables studied on this day. It is concluded that inhibin B concentrations obtained early in the follicular phase during ovarian stimulation under pituitary suppression for assisted reproductive treatment are highly predictive of ovarian response.