The possibility of a correlation between biological (E2) and morphological (U.S.) parameters, in view of obtaining a correct prevision of follicular maturation and ovulation is discussed. In this study normal patients with a tubal sterility were treated with clomiphene citrate and clomiphene plus H.M.G. The statistical analysis was performed by means of the linear correlation (r) and the relative test of significance (Student T.). The data obtained in this study show that notwithstanding that the rising plasma concentration of E2, which precedes ovulation, is the first and more constant marker of follicular maturation, because of its wide range of variability its usefulness in prediction of ovulation is nearly limited to spontaneous cycles. In fact in stimulated cycles the rising plasma-E2 concentration is rather an independent variable regarding the growth of single follicles (F.D.).