['The objectives of this study were: (1) to correlate follicle size by transvaginal sonography with ovulation outcome in cycles of controlled ovarian hyperstimulation with human menopausal gonadotropins; (2) to determine if follicular size on the day of human chorionic gonadotropin (hCG) administration predicts the incidence of ovulation; and, if so, (3) to derive a mathematical model that predicts the number of expected ovulations in any given cycle of controlled ovarian hyperstimulation.', 'A retrospective analysis.', 'Forty-nine consecutive patients undergoing 122 cycles of controlled ovarian hyperstimulation were studied in a tertiary care setting.', 'Follicular size and evidence of ovulation were determined sonographically. The main outcome measure was the rate of ovulation per follicle size.', 'The percentages of follicles measuring less than or equal to 14 mm, 15 to 16 mm, 17 to 18 mm, 19 to 20 mm, and greater than 20 mm on the day of hCG administration that subsequently ovulated were 0.5%, 37.4%, 72.5%, 81.2%, and 95.5%, respectively.'] Does follicular size at the time of human chorionic gonadotropin administration predict ovulation outcome in human menopausal gonadotropin-stimulated cycles?