Background: Leukopenia is a potentially severe side effect of antipsychotic drugs. Clozapine, olanzapine, and chlorpromazine are found to induce leukopenia. Haloperidol-induced leukopenia is occasionally reported. Research are limited to explore hematologic side effects related to the combined use of olanzapine and haloperidol. Case Report: We present a case of a patient with schizophrenia, whose white blood cell count was decreased following receiving a combination of haloperidol and olanzapine, and increased rapidly after discontinuing haloperidol. Conclusion: This case reminds us paying attention to the leukopenic effect of the use of haloperidol or combined antipsychotic drugs. Encountering a patient with a suspected antipsychotics-induced hematologic problem, clinicians need to consider those frequently mentioned offending agents, and to observe its relation with other antipsychotic drugs.