OBJECTIVE: To identify risk factors for multiple pregnancies in intrauterine insemination (IUI) cycles with recombinant follicle-stimulating hormone (r-FSH). STUDY DESIGN: A retrospective study including 205 IUI cycles with r-FSH which led to clinical pregnancies was conducted. A total of 145, singleton pregnancies and 60 multiple pregnancies were compared according to clinical characteristics and parameters of ovarian stimulation and IUI procedure. The relationships between size and number of follicles and serum estradiol (E2) levels and the risk of multiple pregnancies were investigated using multiple logistic regression analyses. RESULTS: The means of infertility length, serum E2 levels, the number of follicles 10 mm, 12 mm, and ≥ 16 mm, and the number of intermediate follicles (from 12 to ≤ 15 mm) at the day of ovulation triggering were significantly higher in the multiple pregnancy group as compared to in the singleton pregnancy group (p < 0.05). We first demonstrated that high E2 levels (≥ 1,000 pg/nL) and the number of intermediate follicles represent 2 independent and significant risk factors for multiple gestation in IUI cycles that used ovarian stimulation by r-FSH (p = 0.002 and p = 0.007, respectively). CONCLUSION: This study shows that high E2 levels and the number of intermediate follicles, independently of large follicles, can predict an increased risk of multiple pregnancy in r-FSH IUI cycles.