OBJECTIVE: To synchronize the intercycle FSH elevation with exogenous E2 for programming ovulation in the menstrual cycle. DESIGN: Open single-arm study. SETTING: Teaching hospital. PATIENT(S): Twenty-six patients with infertility whose menstrual cycles normally lasted 25-35 days and who underwent our routine programming method for postcoital tests and ovulation evaluations. INTERVENTION(S): Participants received estradiol valerate (2 mg) twice a day from day 25 of the previous cycle until 1-15 days after the onset of menses. Women had ultrasonography on the last day of E2 treatment or on functional day 0 and 13 days later or on functional day 13. Hormones were determined on functional days 0, 3, 9, and 13. The increase in FSH in response to E2 withdrawal was defined as deltaFSH. MAIN OUTCOME MEASURE(S): LH surge and other ovulatory indices on functional day 13. RESULT(S): On functional day 13, 73% of the women had an LH surge. Fifteen percent had evidence of previous ovulation with low LH and elevated plasma P levels, and the remaining 12% had low LH levels and no evidence of past or imminent ovulation. Women with evidence of early ovulation were older and had higher FSH signal amplitude. CONCLUSION(S): It is feasible and practical to program ovulation in the menstrual cycle with exogenous E2. In 73% of women, the true duration of the follicular phase (intercycle FSH elevation to LH surge interval) remained constant (13 days). Hence, common fluctuations in menstrual cycle length mainly result from variations in the timing of the intercycle FSH elevation. Although rare, truly short follicular phases also exist (15%). This simple and practical system for programming natural ovulation offers new possibilities for using the menstrual cycle in assisted reproductive technology, at least in selected individuals.