The relative frequencies of non-Hodgkin's lymphoma (NHL) subtypes and primary sites of presentation in young men before and during the acquired immunodeficiency syndrome epidemic were studied. Diagnostic pathological specimens were reviewed and classified according to the Working Formulation grades for cases of NHL in men aged 20-49 years identified through a population-based cancer registry. 76% of the lymphomas diagnosed in 1981-82 were intermediate or high grade, compared with 29% of those diagnosed in 1976-77. This increase was still significant when men aged 20-39 years and 40-49 years were grouped separately, and for married men. The proportion of extranodal lymphomas diagnosed in men aged 20-39 years also rose significantly from the early to the later period.