This study sought to determine the incidence and pattern of occurrence of Waldenstrom's macroglobulinemia, a plasmacytoid lymphocyte malignancy that involves monoclonal production of the IgM M-component type. Cases with Waldenstrom's macroglobulinemia have been reported since 1978 to the population-based cancer registry that serves western Washington state, and since 1988 to the eight other cancer registries that participate in the National Cancer Institute's Surveillance, Epidemiology, and End-Results program. Persons less than 85 years old newly diagnosed with Waldenstrom's macroglobulinemia were identified through 1989. The age-standardized annual incidence rate was 6.1 per million in white men and 2.5 per million in white women (1980 US standard). Only five cases were reported in black women, among whom the age-standardized annual incidence rate was 3.6 per million. No cases were reported among black men (5.8 cases expected, based on the rates in white men); this finding may be due to chance, underdiagnosis of Waldenstrom's macroglobulinemia in this group, or may reflect a truly low rate. Further investigation of a large, racially diverse population is required to better characterize the epidemiology of this rare disease.