Thiamine deficiency is a common feature in chronic alcoholic patients, and its pathophysiology remains poorly understood. Until now, thiamine deficiency has been considered to be mainly the result of alcoholism irrespective of the underlying liver disease. The aims of the study were to compare the prevalence of thiamine deficiency in alcohol- and hepatitis C virus-(HCV-) related cirrhosis and in patients with chronic hepatitis C without cirrhosis. Forty patients with alcoholic cirrhosis (group A), 48 patients with HCV-related cirrhosis (group B), and 59 patients with chronic hepatitis C without cirrhosis (group C) were included prospectively. Thiamine status was evaluated by concomitant determination of erythrocyte transketolase activity, thiamine diphosphate (TDP) effect, and direct measurement of erythrocyte thiamine and its phosphate esters by HPLC. Thiamine was mainly present in erythrocytes in its diphosphorylated form. Prevalence of thiamine deficiency and levels of TDP in thiamine-deficient patients were similar in patients of group A (alcoholic cirrhosis) and of group B (viral C cirrhosis). None of the patients with chronic hepatitis (group C) was deficient. Thiamine deficiency was not correlated with the severity of the liver disease or disease activity. No impairment of thiamine phosphorylation was found in the three groups. conclusion, alcoholic or HCV-related cirrhotics have the same range of thiamine deficiency, while no patient without cirrhosis has thiamine deficiency, and impaired phosphorylation does not account for the deficiency observed in cirrhotics. We suggest that thiamine should be given to patients with cirrhosis irrespective of its cause.