The incidence of colorectal cancer was compared in nine towns in England and Wales, chosen to encompass a range of socio-economic conditions and spread of latitude in the country. Cases were ascertained through pathology records, supplemented by clinical notes. The pattern of variation in incidence was different for men and women. Among men incidences were highest in towns with better socio-economic conditions, whereas among women the trend was reversed. This supports the hypothesis that the dominant aetiological influences causing colorectal cancer differ in the two sexes. Mortality rates did not correlate closely with incidence. This, together with the markedly different patterns of incidence of colorectal cancer and appendicitis in the nine towns, casts doubt on the significance of a reported inverse correlation between regional mortality from colonic cancer and the consumption of pentosic fibre.