OBJECTIVE: It is well documented that clozapine treatment causes agranulocytosis, but it can also induce drowsiness, constipation, and hypersalivation; however, these symptoms are usually less severe. It has been reported that clozapine-treated patients with schizophrenia and psychiatric healthcare professionals consider different side effects to be important. The aim of this study was to assess current practice related to the side effects of clozapine in clozapine-treated patients with schizophrenia and psychiatric healthcare professionals in Japan. METHODS: Data were collected from January 2014 to August 2015 in Okehazama Hospital, Kakamigahara Hospital, and Numazu Chuo Hospital. Clozapine-treated patients with schizophrenia and psychiatric healthcare professionals (psychiatrists and pharmacists) were enrolled in this study. RESULTS: Of the 106 patients and 120 psychiatric healthcare professionals screened, 100 patients and 104 healthcare professionals were included in this study. We asked the patients what side effects caused them trouble and we asked psychiatric healthcare professionals what side effects caused them concern. The patients and psychiatrists held similarly positive views regarding the efficacy of clozapine. The healthcare professionals were concerned about agranulocytosis (92.4%), blood routines (61.3%). On the other hand, the patients experienced hypersalivation (76.0%), sleepiness (51.0%). A positive correlation (R=0.696) was found between patient satisfaction and DAI-10 score. CONCLUSION: Patients experienced more problems than healthcare professionals expected. However, usage experience of clozapine healthcare professionals tended to have similar results to patients. It is necessary that all healthcare professionals fully understand the efficacy and potential side effects of clozapine. This is very important for promoting clozapine treatment in Japan.