Endometrial thickness and texture were evaluated with serial pelvic sonograms in ten normal, menstruating women and 107 patients undergoing ovulation induction for in vitro fertilization. Serial sonograms were performed in the control group every other day throughout the menstrual cycle, whereas the sonographic examinations of the ovulation induction patients were performed daily in a four- to eight-day time interval after their medications were completed and prior to laparoscopy for follicular aspiration. Endometrial thickness increased (defined as a persistent change greater than or equal to 1 mm) in 21 of 53 women (40 percent) who received human menopausal gonadotropin (HMG). Of the 54 women treated with clomiphene citrate (CC), five of 12 (42 percent) at 50 mg daily, 13 of 33 (39 percent) at 100 mg daily, and one of nine (11 percent) at 150 mg daily were found to have endometrial thickening. Although the frequencies of endometrial thickening in the high-dose CC and HMG groups were not significantly different (P = 0.09), the peak estradiol (E2) levels in the CC (150 mg) group (E2 = 944 +/- 165 pg/ml) were significantly higher (P less than 0.001) than those in the HMG group (E2 = 584 +/- 26 pg/ml). The relatively low frequency of endometrial thickening in women receiving CC (150 mg) despite very high E2 levels, and the decreased incidence of endometrial thickening relative to increasing dosages of CC may reflect an antiestrogenic effect of CC on the endometrium. In those patients undergoing ovulation induction, the texture of the endometrium remained the same in 98 cycles and changed during nine.(ABSTRACT TRUNCATED AT 250 WORDS)