Background: In the present study, immunological alterations were investigated as one possible factor contributing towards the pathogenesis of schizophrenia. Specifically cellular changes, deviating cytokine production and interfering variables were studied in order to improve our understanding of how these factors interact. Method: 44 acutely ill schizophrenics were compared with matched healthy controls. Cell numbers were determined by flow cytometry and cytokine production by whole blood assay and ELISA. A criss-cross technique was employed for the assessment of interfering serum factors. Results: Cell counts for leukocytes, lymphocytes, pan T cells, activated T cells and the absolute B cell count of the schizophrenic patients were all within normal limits. The absolute and relative monocyte counts, the number of IL-2 receptor carrying T cells and the relative B cell count were slightly elevated. IL-2 and IFN-γ production were increased while IL-10 production, the sIL-2R and cortisol levels remained unchanged. No interfering serum factors were detected. Conclusion: The deficient production of TH-1 cytokines in schizophrenia is not due either to a changed number of immunocompetent cells or to a counterregulation of the TH-2 cytokine IL-10. Serum factors in in vitro testing are not responsible for the deficient cytokine production.