The atypical antipsychotic clozapine carries a high risk of inducing agranulocytosis. We attempted to investigate whether eosinophilia during clozapine treatment has predictive value for subsequent neutropenia/agranulocytosis. One hundred and seventy-seven patients were studied in a prospective naturalistic design using haloperidol as the reference compound. Clozapine was found to differ from haloperidol in respect to their influence on neutrophil granulocytes. In the clozapine group patients with eosinophilia showed a decrease in neutrophil count (less than 2000/mm3 neutrophil granulocytes) significantly more often than patients without eosinophilia.