Abdominal and vaginal sonographic monitoring were compared in 45 patients undergoing follicular development for the purposes of artificial insemination (N = 22) and in vitro fertilization (IVF; N = 23). Transvaginal sonography recorded a significantly (p less than 0.05) higher number of follicles from cycle day 10 on, compared to the abdominal (sector and linear) approach. Starting on day 8, the size of the dominant follicle was also significantly (p less than 0.05) higher when transvaginal endosonography was used. The number of follicles greater than 15 mm on the day of human chorionic gonadotropin (hCG) administration was correlated with the number of oocytes retrieved in patients undergoing IVF. There was a significant linear correlation when transvaginal (r = 0.86, p less than 0.001) and sector (r = 0.57, p less than 0.05) scans were used.