This is a broad meta-analysis of the relations of both depression and stressors to immunological assays. The number of study samples (greater than 180) and measures (greater than 40) is much more extensive than any so far. Analyses are done by both fixed and random effects. By a fixed-effects analysis, both major depression and naturally occurring acute stressors are associated with (1) an overall leukocytosis, (2) mild reductions in absolute NK-cell counts and relative T-cell proportions, (3) marginal increases in CD4/CD8 ratios, and (4) moderate decreases in T- and NK-cell function. However, the degree of heterogeneity of the studies' results raises questions about their robustness. Therefore, we also did the first random effects analysis to estimate what is likely to appear in future studies. For depression, the analysis showed the immunological correlates included (1) an overall leukocytosis, manifesting as a relative neutrophilia and lymphoenia; (2) increased CD4/CD8 ratios; (3) increased circulating haptoglobin, PGE(2), and IL-6 levels; (4) reduced NK-cell cytotoxicity; and (5) reduced lymphocyte proliferative response to mitogen. For stressors, the random effects analysis showed that future studies are likely to find the following effects: (1) an overall leukocytosis, manifesting as an absolute lymphocytosis; (2) alterations in cytotoxic lymphocyte levels, CD4/CD8 ratios, and natural killer cell cytotoxicity with the direction of change depending on the chronicity of the stressor; (3) a relative reduction of T-cell levels; (3) increased EBV antibody titers; (4) reduced lymphocyte proliferative response and proportion of IL-2r bearing cells following mitogenic stimulation; and (5) increased leukocyte adhesiveness. The random-effects analysis revealed that for both major depression and naturally occurring stressors the following effects are shared: leukocytosis, increased CD4/CD8 ratios, reduced proliferative response to mitogen, and reduced NK cell cytotoxicity. The implications for these findings for disease susceptibility and the pathophysiology of these conditions is discussed. Copyright 2001 Academic Press.