Objectives Severe mental illness has become one of the leading concerns for the cost of health services. This study analyzed the demographic characteristics and compared the patterns of medical health service use according to the diagnosis of severe mental illness, including schizo-phrenia spectrum disease, bipolar disease, and major depressive disorder. Methods The data from the National Health Insurance Corporation were analyzed, selecting subjects diagnosed at least once for severe mental illness between 2014 and 2019. Severe mental illness included the following: schizophrenia, schizotypal, and delusional disorders (F20–29); manic episodes and bipolar affective disorder (F30–31); and moderate depressive episodes with psychotic features and recurrent depressive disorder (F32.3–F33). The demographic factors and patterns of medical services, such as outpatient, hospitalization, and re-admission differenc-es, were compared according to the diagnostic categories. Results This study included 842459 patients, with 39.6% people in F20–F29, 33.7% in F32.3–F33, and 26.8% in the F30–F31 category. There were significant differences in gender, age, insur-ance type, Charlson Comorbidity Index score, and economic level according to the diagnostic categories. The engagement of medical health services also showed significant differences among the diagnostic categories. F32.3–F33 showed higher engagement of outpatients than the out-groups, while F20–F29 showed a higher admission rate. The hospitalization duration was sig-nificantly longer in F20–F29, and the re-admission rate after discharge within one year was signifi-cantly higher in the same group. Conclusion This paper reviewed the differences in medical care utilization among severe men-tal illness. The result emphasizes the need for a mental health care system broadening to the community, facilitating psychosocial intervention, and case management.