The immunization of healthy adults with vaccinia virus (VV) induces a protective response against smallpox in most individuals but is also reactogenic in a significant number of vaccinees. The immunological mechanisms underlying the protective response or adverse events in humans are not well defined. Although cytokines contribute to antiviral immunity and, in some cases, cause systemic adverse effects, their role in the human response to VV is unknown. We investigated the effect of smallpox immunization on systemic cytokine concentrations in a cohort of VV-naive individuals. We found that smallpox immunization induces an interferon (IFN)- gamma -dominant response in the systemic compartment 1 week after immunization, with concentrations returning to baseline during convalescence. The level of IFN- gamma induced was not affected by the dilution of vaccine used. We also found that particular adverse events correlated with systemic cytokine patterns, which suggests a role for these molecules in the pathogenesis of adverse events.