BACKGROUND: In New Jersey, the age-adjusted incidence rate of non-Hodgkin lymphoma (NHL) increased more than a third from 1979 to 1996, the largest increase among the major cancers. METHODS: Data from a linkage of New Jersey's population-based cancer and acquired immune deficiency syndrome (AIDS) registries were used to obtain two sets of annual age specific incidence rates and estimated average annual percentage changes in the incidence rates, for each of five adult age groups within each gender, from Poisson regression models that 1) included all the NHL cases and 2) excluded the cases of AIDS-NHL. RESULTS: During 1979-1996, of the NHL cases aged 15 years and older reported to the cancer registry, 687 (6%) of the 11,725 male cases and 139 (1%) of the 10,785 female cases were AIDS-NHL. The highest percentages of AIDS-NHL were in the younger age groups--15-29, 30-39, and 40-49 years. Among both men and women, average annual percentage increases in NHL occurred overall (3.1 and 3.0, respectively), and in each age group, ranging from 1.6 and 1.9, respectively, in the 50-59 years age group to 6.6 and 4.2, respectively, in the 30-39 years age group (P <<0.01). Excluding AIDS-NHL, the estimated average annual percentage increases in NHL were greatest in the 30-39 and the 60 years and older age groups among men, and these two age groups plus the 15-29 years age group among women, ranging between 2.4 and 2.9 (P < 0.05). CONCLUSIONS: As elsewhere, factors in addition to AIDS are involved in the increasing incidence of NHL in New Jersey. Because diagnostic and classification changes probably do not explain the entire increase unrelated to AIDS, other risk factors are likely responsible. Public health interventions to reduce the incidence of NHL not related to AIDS are problematic until more is known about the causes of NHL.