We conducted a study on 165 women with thyroid cancer and 214 hospital controls in order to investigate the role of reproductive and hormonal factors in the aetiology of thyroid cancer. Late age at menarche (Relative risk (RR) for menarche at greater than or equal to 14 vs less than or equal to 11 = 2.8), menstrual irregularity (RR = 1.7), late age at first birth (RR for first birth at greater than or equal to 28 vs less than or equal to 21 = 2.4) and at last birth (RR for last birth greater than or equal to 30 vs less than 30 = 2.2) significantly increased the risk of the development of thyroid cancer both in premenopausal and postmenopausal women. Parity was, in the present study rather inconsistently related to disease status whereas voluntary abortions and miscarriages were completely unrelated. A non significant increasing risk was observed with age at menopause older than 50, and with the use of oral contraceptives in premenopausal women. The mechanism of action of female hormones on the thyroid gland remains largely obscure, but the observation that age at first and, perhaps, subsequent pregnancies may be relevant points to an interplay of reproductive factors (and, possibly, their hormonal correlates) more complex than previously suggested.