Problems associated with outpatient pharmacotherapy may require hospitalization. However, such hospitalization may be prevented if pharmacist's pharmaceutical care (PC) is given. We investigated the reasons for hospitalization in medical institutions and medical expenses were calculated. Inpatient diagnoses, treatment, etc. in the previous year in the past were examined, and cases of hospitalization due to drug therapy were extracted. Next, the possibility of preventing hospitalization with PC practice was examined. Among 1552 cases, outpatient pharmacotherapy was the reason for hospitalization in 27 cases. Noncompliance was the underlying cause in about 40% of hospitalizations. It was thought that in 22 cases hospitalization could have been prevented by pharmacist's PC. The average hospitalization medical expense was ¥295,805 per patient. It is necessary to perform regular consultation recommendations, interventions with the family, home care, etc. for proactive PC.