Previous glucose improves subsequent glucose tolerance (the Staub-Traugott effect) in normal man. We have investigated whether a small amount of glucose (5 g) given perorally 30 min before breakfast would improve postprandial hyperglycemia in type 2 diabetics (19 patients). Blood glucose was increased 30 min after glucose ingestion (from 7.6 +/- 0.4 to 8.7 +/- 0.4 mmol/l, p less than 0.001). Total glucose areas measured between the time of glucose ingestion and 180 min after breakfast were similar during test and control conditions (breakfast alone). Apparent differences between individuals with regard to the effects of previous glucose on hyperglycemia were further analyzed. Differences could not be explained by interexperimental variation since they persisted on repeated testing (3 patients). Differences were not correlated with age, sex, duration of diabetes, obesity, fasting blood glucose or the insulin responses evoked in the experiments. We conclude that a small amount of glucose before breakfast fails to ameliorate postprandial hyperglycemia in overt type 2 diabetics except in individual patients in whom, in turn, the effect is not directly related to insulin secretion.