Immunologic effects of high-dose interferon are still unclear. We have evaluated changes in blood lymphocyte subpopulations, immunoglobulins, and multiple interleukin in patients with high-risk cutaneous melanoma on adjuvant treatment with high-dose interferon and compared pretreatment values with normal controls. Samples were obtained before treatment, 1 month after induction treatment and at 3, 6, and 12 months of maintenance treatment from 24 patients with high-risk melanoma. Lymphocyte subpopulations were measured by flow cytometry and interleukin and immunoglobulin levels by radioimmunoassay. A statistically significant reduction in B-lymphocytes (p < 0.001), natural killer (NK) cells (p = 0.0004), and monocytes (p = 0.04), and an elevation in CD4/CD8 ratio (p < 0.0001) was observed after 1 month of intravenous interferon. No changes were seen in CD3, CD4, and CD8 lymphocytes. No changes in interleukin (IL)-2, -4, or -5 were observed during 1 year of treatment. IL-2 pretreatment levels were significantly lower than healthy blood donors (p = 0.001), and IL-5 pretreatment levels were significantly higher (p = 0.0056). IL-10 levels significantly dropped after 6 months of treatment (p = 0.01). Immunoglobulins (IgG, IgA, IgM) remained within normal ranges. Three patients had elevated pretreatment levels of IgE. There is a time- and dose-dependent impact of interferon on numbers of circulating B lymphocytes, NK cells, monocytes, and CD4/CD8 ratio. Defects in cellular and humoral immunity are suggested by the low IL-2 and high IL-5 levels, measured in patients with melanoma as compared with healthy controls.