Based on data from the Nordic cancer registries, time-related trends in incidence of Kaposi's sarcoma (KS) were analysed in four ethnically similar populations before the AIDS epidemic. Data were available for different time periods in Denmark (1970-79), Sweden (1958-79), Finland (1953-79) and Norway (1953-79). KS was more common among men than among women aged 60 years or more, whereas no differences were observed for younger persons. The incidence of KS differed significantly between the four countries (P = 0.0001); Sweden having the highest and Denmark the lowest rates. Similarly, regional differences in incidence were observed within Sweden, rates being higher in the northern than in the southern areas (Ptrend = 0.002). Overall, in Nordic men the world standardised incidence rose from 0.5/1,000,000 person-years in the period 1953-57 to 1.8/1,000,000 person-years in 1978-79; in Nordic women, the corresponding rates were 0.2/1,000,000 person-years and 0.8/1,000,000 person-years respectively. The rate of increase was similar in Sweden, Finland and Norway (P = 0.14), whereas the short period of observation in Denmark precluded precise assessment of time-related incidence trends. These observations cannot be explained by registrational procedures or known risk factors for KS, and argue that environmental factors play an important role in the development of the disease.