Differences between the sexes in time trends of colorectal cancer incidence 1962-87 and mortality 1960-91 in England and Wales are examined in relation to changes in female hormonal factors. There was a trend in the sex ratio of this tumour, particularly marked for the descending colon, whereby the female excess in risk at young ages has almost disappeared but the male excess at older ages has increased. This trend started for cohorts born since the 1920s and coincided with the increase in the use of oral contraceptives and, to a lesser extent, with increases in fertility. The decline has been particularly pronounced for women at young ages born since 1935-39, coinciding with the spread of oral contraceptive use to younger age groups. These results are consistent with the hypothesis that female hormonal factors may play a role in the aetiology of colorectal cancer and with the possibility that oral contraceptive use might exert a protective effect in the descending colon.