ObjectiveTo evaluate the effectiveness of a composite oral health promotion program designed to reduce dental plaque among persons with severe mental illness in a psychiatric institution.MethodsA cluster randomized controlled study was carried out in chronic psychiatric wards of a general hospital in central Taiwan. Sixty-eight eligible male individuals admitted to 2 wards were randomly assigned to an experimental and a control group. Participants in the experimental group underwent an oral health promotion program that consisted of biweekly group education sessions, and a 12-week individual behavioral modification for oral hygiene course. The participants in the control group received usual care only. Dental plaque (measured by the Plaque Control Index) was examined by a single dentist before and after the experiment. Each participant responded to a questionnaire regarding oral health knowledge, attitude and behavior before and after the experiment.ResultsFifty-eight individuals completed the study. Before the experiment, the plaque index was similar between the intervention group (68.9; N = 27) and the control group (69.8; N = 31). After the experiment, the plaque index was significantly better in the intervention group than in the control group (42.6 vs. 61.8; P < 0.001). Participants in the intervention group also demonstrated better oral health knowledge, attitude and behavior than those in the control group after the experiment.ConclusionsA composite oral health promotion program using both group education and individual behavioral methods over a 12-week period was effective in both reducing dental plaque and improving the oral health knowledge of persons with severe mental illness in the institution.Trial registration: This study was retrospectively registered in Clinicaltrials.gov, with number NCT04464941, dated 7/7/2020. https://register.clinicaltrials.gov/RD103035018.