The present study was undertaken to determine whether the sonographic characteristics of the endometrium in the proliferative phase of the natural cycle combined with meticulous hormonal monitoring would have an impact on the prediction of conception. Fourteen women with regular ovulatory cycles were examined daily, from day 8 of the cycle, by transvaginal ultrasound through 16 unstimulated cycles. All patients had had at least four previous unsuccessful donor artificial insemination cycles scheduled by basal body temperature records. During the study, donor artificial insemination was meticulously timed by hormonal and sonographic monitoring. Four clinical pregnancies occurred with a success rate of 29% per patient, or 25% per cycle. The endometrial thickness at the mid-proliferative phase (day 8) was significantly smaller in patients who conceived in comparison with patients who failed to conceive (0.33 +/- 0.07 cm vs. 0.59 +/- 0.03 cm; p < 0.01). The endometrial thickness on the day of the luteinizing hormone (LH) peak was not significantly different between the two groups. However, the amount of endometrial growth between day 8 of the cycle and the day of the LH peak was significantly greater in conception than non-conception cycles (0.55 +/- 0.05 cm and 0.24 +/- 0.06 cm; p < 0.01). No difference in hormonal parameters was observed either on day 8 of the cycle or on the day of the LH peak between conception and non-conception cycles. On the day of the LH peak, an endometrium with typical triple line appearance and thickness of 0.6 cm or more was seen significantly more often in conception than non-conception cycles (p < 0.05). No pregnancy occurred with an endometrium < 0.6 cm at the time of ovulation. The presence of the favorable endometrial pattern in the periovulatory phase was associated with a positive predictive value for pregnancy of 50% (sensitivity of 100% and specificity of 67%), whereas the negative predictive value of the hyperechoic pattern at the time of ovulation was 100%. Our data show negative correlation between early growth of the endometrium and pregnancy in unstimulated cycles, suggesting that the amount of endometrial growth during the cycle up to ovulation may have an important role in implantation. The sonographic image of the endometrium in natural cycles provides useful information about the possible outcome of the treated cycle. Copyright 1991 International Society of Ultrasound in Obstetrics and Gynecology