Endovaginal and transabdominal sonography (EVS and TAS, respectively) are effective methods for monitoring ovarian follicular development. Our study evaluates both modalities by subjective comparisons and objective correlations with serum estradiol levels. In 21 of 41 studies, TVS showed more follicles over 1 cm than did TAS. The same number was seen by both in 17 studies and more by TAS in only 3 studies. Margins of the follicles were sharply defined in 37 of 41 EVS studies (90%) but only in 18 of 41 TAS studies (41%). In each of the 4 cases in which follicles were not sharply defined by EVS the ovaries were located high in the pelvis. In 29 sonographic studies, there were corresponding estradiol levels. The number and size of dominant follicles correlated well with estradiol levels (r = .9074) for EVS but less so (r = .3816) for TAS. EVS has several advantages when monitoring follicular development.