HMG treatment is usually monitored by the evaluation of the cervical mucus, the determination of plasma 17 beta-estradiol, total urinary estrogens, ultrasonographic evaluation or a combination of these. We evaluated the daily validity of estrone-3-glucuronide excretion in urine as an indicator of follicular growth and maturation in 28 infertile women who were treated with HMG (Pergonal 500). Total urinary estrogens and estrone-3-glucuronide were measured in 24 h urine collections, and 11 of the women collected the early-morning urine separately. This allowed comparison of the concentrations and excretion of total estrogens and estrone-3-glucuronide of the 24 h urine with that found in the urine collected overnight. This comparison was made on 83 urine samples. The correlation between either the total excretion per 24 h or the concentration per liter in the 24 h urine collection of the two systems of determination was good in all determinations. Also in the urine collected on the day prior to HCG administration, total estrogens measurement was in good correlation with the estrone-3-glucuronide. However, there was statistically a significant difference in the concentrations of total estrogens and estrone-3-glucuronide between the women who ovulated and those who did not. Estrone-3-glucuronide, when calculated as a percentage of the total estrogens, was 60.86% in the women who ovulated and 33.15% in those who did not. These results demonstrate that although estrone-3-glucuronide reflects ovarian function in women treated with HMG, it may serve as a better predictor to ovulation.