CONTEXT: Humor therapy and the related mirthful laughter are suggested to have preventive and healing effects. Although these effects may be mediated by neuroendocrine/neuroimmune modulation, specific neuroimmune parameters have not been fully investigated. OBJECTIVE: To determine the efficacy of mirthful laughter to modulate neuroimmune parameters in normal subjects. DESIGN: A series of 5 separate studies based on a multivariate repeated measures design, with post hoc simple contrast analysis. SETTING: The schools of medicine and public health at Loma Linda University, Loma Linda, Calif. SUBJECTS: 52 healthy men. INTERVENTION: Viewing of a humor video for 1 hour. Blood samples were taken 10 minutes before, 30 minutes into, and 30 minutes and 12 hours after the intervention. MAIN OUTCOME MEASURES: Natural killer cell activity; plasma immunoglobulins; functional phenotypic markers for leukocytes including activated T cells, nonactivated T cells, B cells, natural killer cells, T cells with helper and suppressor markers, and assessment of plasma volume and compartmental shifts; plasma cytokine--interferon-gamma; and total leukocytes with subpopulations of lymphocytes, granulocytes, and monocytes. RESULTS: Increases were found in natural killer cell activity (P < .01); immunoglobulins G (P < .02), A (P < .01), and M (P < .09), with several immunoglobulin effects lasting 12 hours into recovery from initiation of the humor intervention; functional phenotypic markers for leukocyte subsets such as activated T cells (P < .01), active cytotoxic T cells (P < .01), natural killer cells (P = .09), B cells (P < .01), helper T cells (P < .02), uncommitted T cells with helper and suppressor markers (P < .02), helper/suppressor ratio (P = .10) with several leukocyte subset increase effects lasting 12 hours after the humor experience; the cytokine interferon-gamma (P = .02), with increases lasting 12 hours; total leukocytes (P < .05), with specific subpopulation lymphocytes during the intervention (P < .01) and 90 minutes into recovery (P < .05); and granulocytes during the intervention (P < .05) and 90 minutes following the intervention (P < .01). CONCLUSION: Modulation of neuroimmune parameters during and following the humor-associated eustress of laughter may provide beneficial health effects for wellness and a complementary adjunct to whole-person integrative medicine therapies.