OBJECTIVE: To investigate whether the timing of administration of clomiphene citrate (CC) affects hormone levels, follicular recruitment, reproductive end organs, and pregnancy rates. DESIGN: Prospective, randomized, double-blind trial. SETTING: Academic center. PATIENT(S): Twenty-three patients with unexplained infertility. INTERVENTION(S): Twenty-three patients with unexplained infertility underwent 45 cycles of CC and IUI. For each cycle, patients were randomized either to receive 100 mg of CC on days 1-5 and placebo on days 5-9 (study group), or placebo on days 1-5 and CC on days 5-9 (control group). MAIN OUTCOME MEASURE(S): The difference in uterine artery PI, number of follicles, endometrial thickness, and pregnancy rates. RESULT(S): Gonadotropins and E2 levels, as well as uterine artery pulsatility index, were significantly higher in the study group on day 5. In addition, in the study group, a longer time interval existed between finishing CC and IUI (8 versus 6 days; MD = 2 days; 95% CI = 1-3) and the pregnancy rate was higher than in the control group (6 versus 0; OR = 15.1; 95% CI = 1.1-72.4). CONCLUSION(S): Clomiphene citrate commenced on day 1 of the menstrual cycle, rather than day 5, results in more rapid follicular growth, a longer CC-free period before IUI, and higher pregnancy rates. Although methodologically sound, our results should be taken with some degree of caution because they are based on a relatively small number of patients.