Six patients with gross thiamine deficiency found during normal psychiatric practice in England are described. Two surveys of newly admitted psychiatric patients are recounted. A major degree of biochemical deficiency, with or without minimal clinical manifestations, was found. Only three (1%) of 326 patients surveyed had gross clinical deficiency. It is thought that severe thiamine deficiency, though rare in the Western countries, has not been eradicated. Deficiency may cause psychiatric conditions like Wernicke's encephalopathy or may be a secondary feature in mental illness due to anorexia, in reduced food intake, and in poor nutrition. The importance of being aware of the possibility for correct diagnosis is emphasised.