Abstract: Apoptosis has been considered to be involved in schizophrenia. Water channels are modulated just before apoptosis. In the aquaporin family, aquaporin 4 (AQP-4) is most highly expressed in the brain and is supposed to play an important role in a neuronal environment. In this clinical study, we investigated the relationship between the AQP-4 polymorphism and drug response in schizophrenia under the control of the MAOA (monoamine oxidase A) promoter gene. We recruited 91 patients with schizophrenia, and they were randomized to receive olanzapine (n = 44), risperidone (n = 23), or paliperidone (n = 24). Genotyping of AQP-4 and MAOA polymorphisms was done in all patients. Patients with the AQP-4 non-C polymorphism needed a higher dosage of olanzapine for treatment (z = 4.163, P = 0.041), and patients with a short form of the MAOA polymorphism needed a higher dosage of risperidone for treatment (z = 5.124, P = 0.024). Patients who smoked cigarettes needed a higher dosage of olanzapine for treatment (z = 4.905, P = 0.027), but cigarette smoking did not affect the dosage of paliperidone. The AQP-4 polymorphism may have an effect in influencing the dosage of olanzapine. However, the roles of AQP-4 polymorphisms in the blood-brain barrier and different neuroprotective effects need further exploration in future studies.