Time-related trends in the incidence of non-Hodgkin's lymphoma (NHL) in Denmark were analysed for the period 1943-89. A total of 13,822 patients (7565 men and 6257 women) were included in the study. In men, world-standardised incidence rates per 100,000 population increased from 2.5 in 1943-47 to 9.3 in 1988-89. In women, a similar increase was seen, i.e. from 1.9 in 1943-47 to 6.5 per 100,000 population in 1988-89. For all birth cohorts, the male-to-female incidence ratio was highest among young subjects and fell significantly after the age of 29 years. Trends in age-specific incidence were analysed separately for two periods, i.e. 1943-77 and 1978-89, reflecting an early, pre-AIDS period and a later period possibly influenced by AIDS. In both periods, the incidence of NHL increased in all age groups. However, in recent years a noticeable increase in incidence averaging 8% annually was observed in men and women aged 40-49 years. A number of factors including changes in the perception of NHL and in the diagnostic methods available are considered insufficient to explain the observed increase. The remarkable and parallel lime trends observed in young men and women in recent years indicate that factors other than AIDS must be considered.