As part of a series of investigations into the health of Vietnam veterans, we conducted a population-based, case-control study of non-Hodgkin's lymphoma between 1984 and 1988. All men born between 1929 and 1953 and diagnosed as having non-Hodgkin's lymphoma in an area covered by eight cancer registries were considered eligible. Control subjects were identified by random-digit dialing from these same regions and were frequency-matched to men with lymphoma by age. Analyses of 1157 men with pathologically confirmed lymphomas and 1776 control subjects showed that the risk of non-Hodgkin's lymphoma was approximately 50% higher among Vietnam veterans (odds ratio, 1.47; 95% confidence interval, 1.1 to 2.0) compared with men who did not serve in Vietnam. Vietnam veterans were also at higher risk relative to (1) men who had not served in the military, (2) other veterans, and (3) other veterans who served between 1964 and 1972. An analysis of the military histories of the 232 Vietnam veterans suggested that the relative risk (1) increased with length of service in Vietnam (P = .10), and (2) was higher among men in the sea-based Navy than among other veterans (P = .11). Little difference in risk, however, was noted according to dates of service, type of unit, military region, or any other characteristics that may have been associated with the use of Agent Orange. Although the cause remains uncertain, results of this study indicate that the risk of non-Hodgkin's lymphoma is higher among Vietnam veterans than among other men.