Objective: The aim of the study was to evaluate the periodontal health and treatment needs of chronically hospitalized psychiatric patients in Istanbul, Turkey. Method: The subjects' periodontal health was recorded by the CPI (Community Periodontal Index) method. Results: Of the 330 patients examined, 179 (52.5 %) were males and 151 (47.5%) females. The mean age of the patients was 49.2 +/- 11.7 years. The majority (61.8%) was diagnosed with schizophrenia and 30.6% diagnosed with mental retardation. The mean length of hospitalization was 16.0 +/- 10.9 years. Healthy periodontal tissues (CPI 0) were found in 8.8 % of the subjects. Bleeding on probing (CPI 1) was recorded in 6.3%, and dental calculus (CPI 2) in 51.8% of the subjects. These were determined as the worst findings. Altogether, 33% of the subjects had deep periodontal pockets, 14.2% with at least one 4- to 5-mm pocket (CPI 3), and 18.8% with at least one 6-mm pocket (CPI 4). The stepwise logistic regression analysis, between the final CPI score and seven variables including age, gender, psychiatric diagnosis, length of hospitalization, degree of helplessness, tooth brushing habits and smoking, showed that irregular tooth brushing habits and male gender were significant contributors to having a final CPI score of 2 or more. The regression analysis also showed that tooth brushing habits remained as an explanatory variable in CPI 0 coded subjects; helplessness and psychiatric diagnosis (mental retardation) in CPI 2; tooth brushing habits and psychiatric diagnosis (schizophrenia) in CPI 3; and only helplessness in CPI 4. Conclusion: The present study underlines a considerable need for prevention and treatment of periodontal disease among chronic psychiatric patients in Istanbul. Efforts need to be focused above all on raising this population's awareness of the importance of oral hygiene and on early diagnosis of periodontal problems.