The effect on age at breast cancer diagnosis of age at menarche, age at first baby, parity and age at menopause has been determined for 739 unselected patients diagnosed between 1975 and 1980 as having operable breast cancer. Age at diagnosis was significantly and positively associated with ages at menarche, first baby and menopause. The average number of children significantly declined with increasing age at diagnosis. This was largely due to a change in the proportion of patients who were nulliparous (15% in women aged 41-50 years compared to about 30% in those over 60 years). A group of 1,989 normal women whose reproductive history was also collected between 1975 and 1980 showed similar trends between age and age at menarche, age at first baby and parity as the patients. Thus it appears that these reproductive parameters do not alter the time of onset of breast cancer but could be explained by temporal changes in reproductive patterns. There was no significant correlation between age at menopause and age at diagnosis for patients whose age at menopause was no more than 54 years and age at diagnosis no less than 55 years, respectively. It is therefore unlikely that age at menopause affects age at diagnosis. Postmenopausal patients with 3 or more children had a significantly shorter disease-free interval and lower survival rate than those with less children. None of the other parameters was associated with prognosis.