Serum inhibin concentrations, determimed by radioimmunoassay, were measured in women undergoing pituitary suppression with Decapeptyl and subsequently ovarian stimulation with Highly Purified-Metrodin(HP-FSH) to appraise follicular development. Early follicular basal serum inhibin level correlated with the number of oocytes retrieved(r=0.89, n=8, p<0.05). The number of oocytes retrieved showed a significant correlation with serum inhibin level on the day of hCG administration(r=0.73, n=8, p<0.05). The number of mature oocytes showed a significant correlation with serum inhibin level on the day of hCG administration(r=0.73, n=8, p<0.05). These data suggest that: (1) In the early follicular phase, basal serum inhibin may be a valid index to predict ensuing follicular growth : (2) In the preovulatory phase, maximum serum inhibin may be one of the indexes of follicular development during hyperstimulation cycles.