The clinical experience in an IVF/ER program of a newly developed commercial kit (Estradiol enzymatique; BioMerieux) to determine urinary estrogens enzymatically, is reported. The study included 157 cycles, and the main indication for treatment was infertility due to tubal factor. Follicular development was stimulated in most cases by a combination of clomiphene citrate, human menopausal gonadotropin and chorionic gonadotropin. In addition to the previously used method to monitor follicular growth, ultrasound, daily urinary estrogens (estradiol + estrone) levels were assayed. A correlation between the pattern of urinary estrogen and the clinical results was found. In cycles with continuously rising estrogen up to the time of hCG administration a higher rate of in vitro fertilization and clinical pregnancies were obtained as compared to cycles where urinary estrogen declined or levelled off. It is concluded that this enzymatic assay of urinary estrogens is a good alternative to increase the precision of follicle monitoring when radioimmunological assay of serum estradiol levels cannot be obtained on a daily routine basis.