The simultaneous capillary and venous blood glucose concentrations were measured during 36 oral glucose tolerance tests performed in 36 postmenopausal women. Three of the subjects had chemical diabetes mellitus. In samples obtained before and 120-180 min after the glucose load the differences between capillary and venous blood glucose concentrations were low, whereas samples taken after 15-90 min showed a mean capillary-venous difference of 1.8 mmol/l. This is higher than previously stated by the World Health Organization and the British Diabetes Association. If the definitions that were recommended by these two bodies are used for defining whether the result of an oral glucose tolerance test is to be considered 'normal' or 'abnormal', the present results indicate that the verdict will in some cases be influenced by the route by which the blood was obtained.