Serum levels of interleukin-2 soluble receptor alpha (IL-2sR alpha), interleukin-6 (IL-6) and interleukin-1 receptor antagonist (IL-1ra) were determined both before and during neuroleptic administration in an 8-week treatment protocol for schizophrenia. In comparison with a control group, schizophrenia patients showed significantly higher serum levels of IL-2sR alpha, IL-6 and IL-1ra at weeks 0, 1, 4 and 8, and there was a significant negative correlation between the serum level of IL-2sR alpha at week 1 and the age at illness onset. Those of the schizophrenia patients who were neuroleptic-naive had significantly higher pretreatment serum levels of IL-2sR alpha, IL-6 and IL-1ra than the controls. There were significant positive correlations between the IL-2sR alpha levels at weeks 0 and 1, and the psychopathology scores, evaluated using the positive and negative syndrome scale at week 4. IL-6 levels at weeks 0, 1 and 4 were significantly and positively correlated with the duration of illness. The IL-1ra level at week 1 was significantly and positively correlated with positive symptoms at week 1. The present study supports the suggestion that changes in the immune system are involved in the pathophysiology of schizophrenia.