Schizophrenia has often been observed to be associated with alterations of the cellular immune system. In this study we monitored numerous immune parameters in the course of antipsychotic treatment. 40 patients diagnosed with an acute exacerbation of schizophrenia were tested before and during treatment with antipsychotics. The percentual distribution of lymphocyte subgroups (CD3, CD4, CD8, CD19, CD5, CD16/56, CD25, HLA-DR, gd, CD11a, CD11b and CD49d) was measured by FACS analysis. 20 healthy volunteers served as controls. In the acute state of psychosis a significant elevation of the B-lymphocyte fraction was observed, while the percentage of T-lymphocytes was decreased. These values levelled to those of the control group in the course of treatment. Natural killer cells, integrine expressing cells and CD5+ B-cells showed alterations in numbers after several months of treatment. The latter may be a side effect of antipsychotic treatment. The findings in the acute state of schizophrenia could be seen as a nonspecific stress reaction or an effect of the altered metabolism of neurotransmitters. On the other hand the observed changes in the immune system could be due to infectious diseases or autoimmune processes provoking the symptoms of schizophrenia.