OBJECTIVE Deficiencies of cobalamin and folate may play a causal role in the development or exacerbation of psychiatric illnesses. We compared cobalamin and folate levels in newly admitted psychiatric patients to mentally healthy controls and assessed their correlation with various psychiatric conditions. METHODS All patients consecutively admitted to a psychiatric hospital were examined for serum cobalamin and folate levels. Controls were obtained from a population with no known mental illness. Values were considered to be below normal if cobalamin was <223 pg/ml and folate <3.1 ng/ml. RESULTS The 224 newly admitted patients did not differ significantly from controls, both with regard to the mean cobalamin level and to the prevalence of lower than normal levels. About 30% of patients had low folate values compared to 2.5% in the control group (P<0.0001). Mean folate level in controls was significantly higher than in patients (P<0.0001), where a positive correlation was found between low folate levels and depression. CONCLUSIONS The results of our study suggest that folate levels be assessed in patients admitted to psychiatric wards, especially in those with depression. Further study is needed to evaluate the role of folate and cobalamin in psychiatric illness.