The incidence of lymphoma in the US and Puerto Rico among 13,600 patients is reported by histologic subtypes according to the Working Formulation. The most frequent histologic types were intermediate grades (five per 100,000 among whites, three per 100,000 among blacks, and two per 100,000 among Puerto Rican Hispanics). Low-grade types were next in frequency (2.7 per 100,000 in whites, 1.5 per 100,000 in blacks, and one per 100,000 in Puerto Rican Hispanics). More than 95% of patients had low-or intermediate-grade lymphomas, and of these, intermediate-grade lymphomas occurred in 65% of patients. High-grade types were infrequent in all ethnic groups. The incidence among men was significantly greater than among women in all ethnic groups. There was a distinct peak for small noncleaved cell type among white boys but not girls. For all other histologic types, there was a conspicuous absence of a young adult component, with incidence increasing steadily with age. No evidence for seasonal fluctuation in month of diagnosis was found. This delineation of incidence by histologic groupings, sex, and ethnic group will facilitate future studies using the Working Formulation.