In vitro and preclinical studies show that biochemical and behavioral effects of olanzapine are quite similar to those of clozapine. In recent years, some cases of reported agranulocytosis due to olanzapine have been published. However, none of these studies compared the hematological and immune parameters before and after treatment. The present study is aimed at investigating the influence of olanzapine on the immune cell parameters by comparing these before and in the third month of olanzapine treatment in patients of schizophrenia. Twenty patients who were diagnosed as schizophrenic depending on the DSM-IV diagnostic criteria were included in the study. The immune parameters of patients were compared by measuring them before the treatment and 3 months after treatment. Immune parameters were analyzed by using flow-cytometry equipment labeled Coulter Epics Elite ESP. The positivity of cell-surface antibody was evaluated as percentage. The rates of CD8 in the third month of the treatment were considerably increased relative to pretreatment. Furthermore, rates of CD4/CD8 were significantly decreased in the third month of the treatment relative to before treatment. These findings suggest that immune impairment may occur during olanzapine treatment in patients with. schizophrenia. (C) 2003 Elsevier Science Inc. All rights reserved.