A study of breast cancer in Irish women.

In a study of 100 women with breast cancer and 200 unaffected women the epidemiological characteristic of breast cancer most commonly described--namely, an excess of nonparous women among cases when compared with controls, was not observed. It is suggested that two factors, a dearth of first births born to mothers under 20 years of age and an unusually high proportion of first births when aged 30 years or more, are responsible for this finding. No association was found between breast cancer and socioeconomic status, marital status, age at marriage, age at first pregnancy, parity, or age at menarche. Significant associations with breast cancer included age at natural menopause, hysterectomy, and breast trauma. These findings are discussed.