Subtype-specific incidence patterns in populations at high risk of lymphoma offer insight into lymphomagenesis. The incidence profiles for the 2 most common non-Hodgkin lymphoma subtypes were compared for 2 immunodeficient populations, adults receiving a kidney transplant 1982-2003 (n = 7,730) or diagnosed with human immunodeficiency virus (HIV) infection 1982-2004 (n = 17,175). National, population-based registries were linked and standardized incidence ratios (SIRs) were computed for each cohort and lymphoma subtype. Risk of diffuse large B-cell lymphoma was significantly increased after transplantation (SIR 17.83, 95% CI: 13.61-22.95) and after HIV infection (SIR 58.81, 95% CI: 52.59-65.56). Rates of follicular lymphoma (FL) were neither significantly increased nor decreased in transplant recipients (SIR 0.82, 95% CI: 0.10-2.96) and in people with HIV (SIR 1.25, 95% CI: 0.41-2.91). The findings argue against an infectious or other immunodeficiency-related etiology for FL and clearly differentiate it from diffuse large B-cell lymphoma.