Urban and rural cancer incidence in Denmark in 1943-1987 was analysed. A consistent urban excess was found for all sites combined for individuals of each sex, irrespective of age at diagnosis. The capital:rural incidence ratio was 1.42 for men and 1.25 for women, and these ratios were not affected to any great extent using another definition of urban areas. Urban:rural ratios were highest for cancers of the respiratory, urinary and upper digestive tracts. The differences cannot be explained by tobacco and alcohol consumption alone. Other risk factors linked to urbanisation may contribute importantly to the "urban factor", and analytical studies of data at an individual level are required to establish their relative importance. Our findings contradict the generally accepted view that there is no urban-rural difference in cancer incidence in the relatively small, homogeneous population of Denmark.