A study of 1,478 consecutive in vitro fertilization (IVF) cycles was made to determine if basal follicle-stimulating hormone (FSH) levels and age were independent predictors of IVF performance. Regression analyses indicated independent contributions of both basal FSH and age in predicting cancellation rate, peak estradiol, number of oocytes retrieved, fertilized, and transferred, and total and ongoing pregnancy rates. Miscarriage rate was unrelated to both age and basal FSH. Follicle-stimulating hormone level was a better predictor than age for all outcome variables examined and remained a significant predictor after accounting for age, etiology of infertility, and semen quality. The combined use of age and basal FSH in counseling patients improves the accuracy of prognosis, and may provide an index of functional ovarian reserve ("ovarian age").