The erythrocyte transketolase activity, and the effect of thiamin pyrophosphate stimulation (TPP effect) on this enzyme, was determined in 263 adult Ghanaian subjects who included normal males and females, male labourers known to consume a diet high in carbohydrate, diabetics and pregnant women. Using the guideline that a TPP effect of greater than 25% indicates thiamin deficiency, it was found that a significant proportion of apparently healthy subjects (36%-47%) may be regarded as thiamin deficient. The results are discussed in the light of similar studies carried out by other workers.