ABSTRACT BACKGROUND: The underlying cause of seasonal infertility in humans is unclear, but is likely to be multifactorial. OBJECTIVE: The aim of our study was to compare the pregnancy rates among infertile women who underwent induced ovulation and intrauterine insemination (IUI) with the season in which the fertility treatment was performed. DESIGN AND SETTING: This retrospective cohort study was conducted on 466 patients who were treated in the reproductive endocrinology and infertility outpatient clinic of a tertiary-level women’s healthcare and maternity hospital. METHODS: Retrospective demographic, hormonal and ultrasonographic data were obtained from the patients’ medical records. Clomiphene citrate or gonadotropin medications were used for induced ovulation. The patients were divided into four groups according to the season (spring, winter, autumn and summer) in which fertility treatment was received. Clinical pregnancy rates were calculated and compared between these four groups. RESULTS: There were no significant differences between the seasonal groups in terms of age, infertility type, ovarian reserve tests, duration of infertility, medications used or length of stimulation. A total of 337 patients (72.3%) were treated with clomiphene citrate and 129 (27.7%) with gonadotropin; no significant difference between these two groups was observed. The clinical pregnancy rates for the spring, winter, autumn and summer groups were 15.6% (n = 24), 8.6% (n = 9), 11.5% (n = 13) and 7.4% (n = 7), respectively (P = 0.174). CONCLUSIONS: Although the spring group had the highest pregnancy rate, the rates of successful IUI did not differ significantly between the seasonal groups.