The influence of endogenous gonadal steroids on the urethral pressure profile (UPP) was studied in 27 healthy nulliparous women with the normal ovulatory cycles. The UPP and hormonal values were recorded in the follicular phase, at midcycle, and early and late in the luteal phase. Simultaneous urethrocystometry was performed by means of the microtransducer technique, according to a precise standardized methodology. The parameters of the UPP, as defined by the Standardization Committee of the International Continence Society, and the area under the urethral closure pressure curve (integrated pressure) were calculated. Reproducibility of the UPP measurements was evaluated. In addition to a basal body temperature (BBT) chart, serum levels of 17 beta-estradiol (E2), progesterone (P), follicle-stimulating hormone (FSH), luteinizing hormone (LH), and prolactin were measured at each study session in order to confirm a normal ovulatory pattern for that cycle and for comparison with the urodynamic parameters. The mean values and standard deviation of the UPP parameters in healthy nulliparous women in the supine and sitting positions are presented. Values of the pressure measurements obtained during one menstrual cycle did not change systemically with the hormonal alterations. At midcycle and early in the luteal phase, however, and anatomic and functional lengths of the urethra were consistently increased over those found in the early follicular and late luteal phases. The data suggest a causal relationship between the changes in serum E2 concentrations and the changes in urethral length.