['Deficiency of vitamin B12, a key component in the catabolism of monoamines, is associated with various neuropsychiatric disorders and may be more frequent in hospitalized patients.', 'We reviewed vitamin B12 assays performed in a laboratory of a large Israeli psychiatric hospital over a 23-month period to examine prevalence of low values and compared vitamin B12 deficient patients to those with normal levels on various parameters. In addition, vitamin levels in a random sample of in-patients whose nutritional intake was determined, were examined.', '20% of 644 vitamin B12 assays were in the low (200 pg/ml) and 10% in the deficient (< 160 pg/ml) range. 24 selected vitamin B12 deficient patients (70.8% with diagnosis of schizophrenia) did not differ from controls (N = 35) in age, sex ratio, hemoglobin concentration, MCV, diagnostic distribution or number and length of hospitalizations, but had slightly lower (but normal) mean folate levels. Rates of vitamin B12 deficiency in the patient sample, whose nutritional intake was adequate, did not differ significantly from those in the laboratory survey.'] Are vitamin B12 levels low in hospitalized psychiatric patients?