OBJECTIVE: To investigate a possible linkage between the use of fertility drugs for infertility and the risk of breast and ovarian cancers. DESIGN: Long-term, historic-prospective study. SETTING: Fertility clinic in a university hospital. PATIENT(S): Files of 1,197 infertile women with a mean (+/- SD) follow-up of 17.9+/-5 years (21,407 person-years) were reviewed. Diagnoses, number of courses, and dosage of fertility drugs were extracted from the files. INTERVENTION(S): Cancers were identified by record linkage to the National Cancer Registry. Histopathologic reports and data on estrogen and progesterone receptors in breast cancer tissue were also reviewed. MAIN OUTCOME MEASURE(S): Standardized incidence ratio with 95% confidence interval (CI) were used for risk assessment. RESULT(S): Of 20 breast cancers (standardized incidence ratio, 1.40 [95% CI, 0.83-2.10]), 16 were detected among 780 women who had been exposed to 3,978 cycles of clomiphene citrate (CC) and/or hMG (standardized incidence ratio, 1.65 [95% CI, 0.94-2.68]). The standardized incidence ratio for this cancer was significantly increased only in patients with one or two CC treatments and a dose of