The concentrations of thiamin and thiamin phosphate esters were determined by high performance liquid chromatography in four patients with clinical Wernicke encephalopathy. Three were alcohol abusers, and one had prolonged vomiting and anorexia. Thiamin and thiamin monophosphate were assessed in plasma and whole blood (four patients) and in cerebrospinal fluid (two patients) before and during thiamin treatment. Thiamin diphosphate was also assessed in whole blood in the four patients. Before treatment, thiamin monophosphate was significantly decreased in all patients, and thiamin diphosphate in three. A poor increase in thiamin mono- and diphosphate was paralleled by a slow clinical improvement in one patient, while an increase in all thiamin compounds was observed in two patients with a rapid recovery. Thiamin monophosphate was a more sensitive marker of deficiency than thiamin diphosphate and unphosphorylated thiamin.