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OBJECTIVE--To study long term trends in incidence of anal cancer in a well monitored, unselected population. DESIGN--Descriptive epidemiological study based on data from the Danish Cancer Registry. SETTING--Denmark, 1943-87. MAIN OUTCOME MEASURES--Time related changes in anal cancer incidence according to sex, age, birth cohort, urban or rural residence, and marital status. RESULTS--The incidence of anal cancer remained fairly constant in the period 1943-57 and was similar for men and women, but it increased 1.5-fold among men and nearly tripled among women thereafter. Among men the incidence increased from 0.25 per 100,000 population (world standardised) in 1958-62 to 0.38 in 1983-7 (p = 0.01) and among women from 0.28 to 0.74 (p < 0.01). The greatest increase was among residents of the capital (Copenhagen). During 1943-87 age specific trends increased in young and middle aged men and in all age groups among women. Men with anal cancer were significantly more likely throughout the study period to be unmarried than were patients with cancer of the colon (adjusted odds ratio 2.7; 95% confidence interval 2.0 to 3.6) and stomach (2.1; 1.5 to 2.8), but no association with marital status was found among women. CONCLUSIONS--The distribution and incidence of anal cancer have changed appreciably since around 1960, especially among women, which indicates important aetiological changes. Changes in sexual behaviour may have facilitated the spread of a transmittable agent of aetiological importance. It has recently been suggested that cigarette smoking promotes anal cancer, and this finds indirect support in the synchronism between changes in anal cancer incidence and heavy smoking behaviour. Factors associated with homosexuality are likely to explain some of the cases among men.