Erythrocyte transketolase activity and the effect of adding thiamine pyrophosphate (% TPP effect) were measured in subjects suffering from Wernicke-Korsakoff syndrome both before and during treatment with thiamine and/or thiamine tetrahydrofurfuryldisulphide (TTFD). Transketolase activity was significantly lower in untreated patients than in healthy volunteers. Treatment with either thiamine or with TTFD restored enzyme levels to control values but TTFD produced a greater increase than thiamine in enzyme activity. In a group of seven patients there was no correlation between duration of TTFD therapy and either increase in erythrocyte transketolase activity or % decrease in the TPP effect. However, when three patients were followed at intervals during treatment with TTFD, their erythrocyte transketolase increased progressively. Neither thiamine nor TTFD produced clinical improvement in the mental symptoms of Wernicke-Korsakoff psychosis unless administered early in the course of the disease.