OBJECTIVES: Epidemiological and clinical studies have suggested that low cholesterol levels or clinical trals to reduce cholesterol concentrations may be associated with suicide, violent behavior or depression. The aim of the present study was to determined i) whether suicidal psychiatric patients is characterized by decreased serum cholesterol concentration ; ii) whether significant difference of cholesterol levels might be present according to the psychiatric diagnosis, and iii) whether significant association between suicide severity and cholesterol levels might be present. METHOD: The subjects were 102 psychiatric patients who were admitted to emergency ward following an attempted suicide during the period from January 1994 to July 1997 and 102 age, and sex matched psychiatric controls who were consecutively admitted to a psychiatric ward during the same period, and 102 age, sex matched healthy normal controls. The suicide attempters were divided into 5 grades according to the suicide severity. Serum cholesterol concentrations were measured by a enzymatic method. RESULTS: The serum cholesterol level in suicidal attempters were found to be significantly lower compared with both psychiatric and normal controls. This significant relationship between suicidal attempt and low cholesterol level was observed only in depressive patients, but not in schizophrenics or personality disorder patients. Low cholesterol was significantly associated with the severity of the suicide. CONCLUSIONS: These results support the previous finding that low cholesterol level might be associated with an increased risk of suicide. The fact that the significant relationship was observed only in depressive disorder, but not in schizophrenia or personality disorder raises the possibility that the association between low serum cholesterol and suicidal behavior may have relevance to biological mechanisms in depression. It is hypothesized that low cholesterol levels would be associated with depression by modifying the serotonin, the production of interleukin 2 and melatonin metabolism.