OBJECTIVE: The intercycle FSH signal that initiates follicular recruitment and marks the functional onset of the menstrual cycle is of small amplitude and while it commonly occurs on cycle day 3, this often varies. Hence, its identification and measurement in serum (sFSH) requires serial daily samplings. We attempted to determine whether urine measurements of FSH (uFSH) could offer a non-invasive alternative, using a model where the intercycle FSH signal is controlled by timely use of exogenous E2. PATIENTS AND METHODS: Pilot prospective trial in 21 infertile women having received E2, from day 25 of the previous cycle until the 1st Friday after menses. Blood and first void urine samples were collected, starting on the last day of E2 (baseline) for assessing FSH and creatinin. A sonogram was performed for identification of maturing follicles (>12 mm). RESULTS: uFSH and uFSH/Cr showed good correlation with sFSH (R = 0.52 and 0.63, P < 0.0001 and P < 0.0001, respectively). In 15/21 patients who had an intercycle sFSH elevation, this was confirmed by uFSH elevation, both occurring within 2-4 days after stopping E2. In all these women, the sonogram showed evidence of impending ovulation. The amplitude of the uFSH signal was on average 3 times higher than its sFSH counterpart. In 6/21 women, no intercycle FSH elevation was detected and no ovulation occurred. DISCUSSION AND CONCLUSION: Our results show that the intercycle FSH signal can easily be identified and measured in urine. This novel approach permits more precise assessments of ovarian physiology than with blood measurements.