['Day 5 serum inhibin B during IVF treatment has been investigated as a predictor of outcome.', 'A total of 54 women (< or = 39 years, normal menses and endocrine profiles) were treated with urinary gonadotrophins or recombinant FSH following pituitary down-regulation. Serum day 3 FSH in a preceding cycle was <8.5 IU/l. Plasma inhibin B, inhibin A and estradiol were determined after 4 days of gonadotrophin administration (day 5).', 'Day 5 inhibin B was the most highly correlated with the number of mature follicles (>14 mm), oocytes retrieved and fertilized. Receiver operating characteristic analysis gave high accuracy for day 5 inhibin B in predicting ovarian response and indicated that a threshold of 400 pg/ml may be helpful in the decision as to whether to continue treatment. Women with <400 pg/ml (n = 16) had lower numbers of follicles, mature follicles, oocytes retrieved, fertilized and cleaved compared with those >400 pg/ml (n = 36) and this threshold gave a positive likelihood ratio of 30, 92.9% sensitivity, 95.0% specificity and 86.7% positive predictive value to detect poor ovarian response. Day 5 inhibin B was the best predictor of pregnancy (no live births and four cycles cancelled, low inhibin group; nine live births and no cancelled cycles, high inhibin group).'] Are day 5 inhibin B levels in a treatment cycle predictive of IVF outcome?