Context: Basal FSH is a marker for ovarian reserve. Objectives: The objective of the study was to investigate the predictive value of basal FSH on spontaneous ongoing pregnancy in subfertile ovulatory women. Design: This was a prospective cohort study. Setting: The study was conducted in 19 fertility centers in The Netherlands. Participants: Subfertile ovulatory women without two-sided tubal pathology and in whom the man had normal sperm parameters (total motile count ≥ 3 × 106) participated in the study. Interventions: Interventions included a fertility work-up, including a basal FSH measurement on cycle d 3. Main Outcome Measures: Spontaneous ongoing pregnancy was measured. Results: We included 3519 consecutive couples of which 562 (16%) had a spontaneous ongoing pregnancy within 1 yr. Basal FSH levels of 8 IU/liter or higher were associated with a decreased probability of spontaneous ongoing pregnancy [hazard ratio (HR) 0.93/IU·liter (95% confidence interval [CI] 0.87-0.98)]. In a multivariable analysis, female age (HR 0.97/yr, 95% CI 0.95-0.99), cycle length (HR 0.96/d, 95% CI 0.93-1.0), and FSH levels 8 IU/liter or greater (HR 0.93/IU·liter, 95% CI 0.87-0.99) were strong negative predictors for spontaneous ongoing pregnancy. Addition of FSH to a prediction model based on female age, duration of subfertility, previous pregnancy, referral status, and semen analysis changed the probability to conceive spontaneously from 30% or greater to less than 30% in 97 of 3219 couples (3.0%). Conclusions: In ovulatory women, a basal FSH level of 8 IU/liter or higher is associated with decreasing fecundity, independent of female age and cycle length. Because the number of couples in whom the FSH level alters management decisions is low, we do not recommend routine testing of basal FSH in subfertile couples.