Background: The oral hygiene of patients with a mental illness is an important concern in psychiatric care, and it is necessary to increase the level of self-care among these patients. In this study, we administered an oral care questionnaire to people with mental illness in Japan and compared their answers before (baseline) and at 1 week and 1, 3 and 6 months after they participated in an educational program. Methods: The questionnaire was distributed to 390 patients. It included questions about age, education, income, between-meal snacks, number of teeth, frequency of tooth brushing, and other items. The educational program was developed for the purposes of improving self-care. Results: Before the program, the proportion of male patients who had had a mental illness for >= 10 years was significantly higher among those patients who did not brush their teeth before bed. In addition, such patients did not have primary care dentists, and a significantly higher proportion of male patients, compared with female patients, did not undergo routine dental checkups more than once per year. The educational program resulted in an improvement in the use of fluoride toothpaste from baseline to 6 months after the intervention (p = 0.001). The daily use of interdental brushes or floss was significantly different 6 months after the intervention. Conclusions: Male and long-term inpatients need oral hygiene instructions. Our educational program showed the effects of using oral hygiene tools. Future studies should include a control group to measure the impact of the educational program.