This study was designed for an oral prophylaxis program to remedy the oral condition of the mentally subnormal patient who required treatment at an open type mental hospital. The subjects consisted of 115 inpatients. The mean age was 45.5 years. The mean number of teeth functioning in mastication was 19.6. In order to improve their oral health they were given guidance and their oral hygiene condition was inspected every month. "STPI" was applied to investigate the toothbrush behavior in these patients. After six months, oral condition improved. Relationship between toothbrush behavior and mental illness was analyzed by principle component analysis and rotated by normal valimax method. Four factors were designed as follows. (1) Depressional state; (2) Personality disorders; (3) Hypochondric factor; (4) Manic state. Dental health of chronically mentally ill patients related to suppressive toothbrush behavior. The other hand the findings of this study showed that it was possible to improve their oral health by improving their living condition.