Psychiatric patients are frequently screened for vitamin B12 deficiency in the absence of hematologic or other neurologic findings. To determine the yield of this practice, 162 psychiatric inpatients were screened for vitamin B12 deficiency. Ten patients had initial low serum vitamin B12 levels, but only two had definite B12 deficiency on further evaluation. Three patients who had initially low B12 levels had normal levels subsequently during outpatient follow-up. When low serum vitamin B12 levels are discovered in psychiatric patients without hematologic or neurologic findings, a diagnosis of B12 deficiency should not be presumed without further evaluation. Key words: screening: psychiatric patients; vitamin B12.