OBJECTIVE: A pattern of clinical, behavioral, and experimental findings suggests that individual differences in aggressive behavior may be related to immunologic processes. We evaluated two conflicting models of the relationship: 1) A positive association stems from an adaptive mechanism protecting aggressive individuals from increased exposure to immune stimuli and 2) a negative association is due to potential immunosuppressive effects of high testosterone levels. METHODS: We investigated the models using enumerative measures of cellular and humoral immunity in a sample of 4415 men aged 30 to 48 years who were interviewed and underwent a medical examination. RESULTS: Analysis revealed positive (and curvilinear) associations between aggressive behavior and enumerative measures of helper/inducer and suppressor/cytolytic T lymphocytes and B lymphocytes. The aggression-immunity relationship was independent of testosterone level, age, current health status, and negative health behaviors and was most pronounced for helper/inducer T cells. There was no evidence of a negative association between testosterone and any immune measure. CONCLUSIONS: In a large sample of men, individual differences in aggressive behavior were positively associated with enumerative measures of cellular immunity.