The thiamin status of early bronchial and breast carcinomas has been assessed using "thiamin pyrophosphate" (TPP) stimulating effect of transketolase enzyme activity. The TPP stimulating effect was found to be greater in a high proportion of the cancer patients than in normal subjects, suggesting that these patients may be at risk of thiamin deficiency. The urinary excretory level of thiamin was high in the cancer patients; it is, therefore, unlikely that the biochemical deficiency of the vitamin was due to lack of the vitamin intake. It is, however, possible that the cancer patients were unable to convert thiamin to thiamin pyrophosphate, the active form of the vitamin.