Time related trends in incidence of non-Hodgkin's lymphoma in Denmark in the period 1943-1989 were analysed. A total of 13,822 patients were included in the study. World standardized incidence rates per 100,000 population increased from 2.5 in men and 1.9 in women in 1943-1947 to 9.3 in men and 6.5 in women in 1988-1989. Trends in age-specific incidence were analysed for two periods, i.e. 1943-1977 and 1978-1989. In both periods, the incidence of NHL increased in all age-groups. In general, the rate of increase was higher in the recent period, with the exception of the oldest age-group among whom the incidence increased by 7% annually between 1943 and 1962. In recent years an increase in incidence averaging 8% annually was observed in men and women aged 40 to 49 years. The remarkable and parallel time trends observed in young men and women in recent years indicate that factors other than AIDS must be contemplated.