The incidence of cancer, or the mortality attributed to it, has been compared in urban and rural residents in 13 populations. In each case, the incidence (or mortality) has been higher in the urban areas in each sex, the ratios varying from a minimum of 1.03 to I in men in Japan to 1.63 to I in men in Denmark. Examination of 26 separate types of cancer showed that 23 tended to be more common in towns, I (myeloma) to be evenly distributed, and 2 (cancers of the lip and eye) to be more common in the countryside. The urban excess was greatest for cancers of the bladder, larynx, liver, lung, mouth and pharynx, and oesophagus, and least for leukaemia and non-Hodgkin's lymphoma. It is concluded that differences in personal behaviour (cigarette smoking, alcohol consumption, sexual promiscuity, exposure to ultraviolet light, type of diet, and family size) are the principal factors responsible for the urban excess. Other factors include general atmospheric pollution, occupational hazards, genetic differences in susceptibility, and artefacts of diagnosis and recording. The rural excess was marked for cancer of the lip in both sexes, but less marked and clearly evident only in men for cancer of the eye. Three-quarters of eye cancers are melanomas and the excess incidence in rural areas provides some weak support for the idea that exposure to sunlight contributes to the production of the disease.