Incidence trends in non-Hodgkin's lymphomas for the period 1973-1991 were studied using data from Spain's Zaragoza and Navarre registries. The overall rate of increase was 5.8% per year. Age-period-cohort models were used, also including sex and registry as explanatory variables. In these models, the variable period was categorized according to the observed changes in diagnostic availability during the length of the study. Non-Hodgkin's lymphomas increased with age, and rates were 31% lower in women. Incidence was almost 50% higher in Navarre, but differences in registry procedures might account for this discrepancy. The rise affects all adult age groups and seems to be the result of changes related not only to the period of diagnosis, mainly ascribable to improvements in diagnostic and coding practices, but also to the birth cohort. According to the model, the risk increased with on-coming generations at a rate of 1.5% per year. The AIDS epidemic in Spain is unable to explain this trend, although it may well exert a stronger influence in the future. Additional knowledge is required to understand the observed rise and to prevent the growing numbers of cases that are expected in years to come.