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Objective: To evaluate the effect of follicular phase length (FPL) on pregnancy outcomes and endometrial thickness (ET) among women with unexplained infertility undergoing ovarian stimulation with intrauterine insemination (OS-IUI) with clomiphene citrate, letrozole, or gonadotropins. Design: Cohort analysis of the Reproductive Medicine Network's Assessment of Multiple Intrauterine Gestations from Ovarian Stimulation randomized controlled trial. Setting: Multicenter randomized controlled trial. Patient(s): A total of 869 couples with unexplained infertility who underwent OS-IUI treatment cycles as part of the Assessment of Multiple Intrauterine Gestations from Ovarian Stimulation study. Intervention(s): FPL was evaluated as a categorical variable defined by quintiles (q1: ≤11 days, q2: 12 days, q3: 13 days, q4: 14–15 days, and q5: ≥16 days). Main outcome measure(s): Clinical pregnancy, live birth rates, and ET. Result(s): Decreasing FPL quintiles did not reduce clinical pregnancy or live birth rates in unadjusted or adjusted models with all treatment groups combined or when stratified by the ovarian stimulation medication. All FPL categories had significantly thinner ET compared with the 5th quintile (≥16 days) among women treated with clomiphene citrate or letrozole. Similar but diminished associations were observed among women who underwent ovarian stimulation with gonadotropins, but the observed differences were limited to those with FPL of 12 days or shorter when compared with FPL ≥16 days. Conclusion(s): Although shorter FPL was associated with reduced ET, it was not associated with the outcomes of clinical pregnancy or live birth in women with unexplained infertility undergoing OS-IUI in all treatment groups combined. Similar patterns existed when analyses of clinical pregnancy and live birth rates were stratified by treatment. Clinical trial registration: NCT01044862.