A study was made of serum glucose response to oral glucose challenge in 88,000 white, 12,000 black, and 4,300 oriental adults undertaking routine health checkups. It was found that serum glucose levels one hour after glucose ingestion were markedly lower in the black group than was the case for the whites and orientals. Glycosuria at one hour was also less common in the blacks than in the whites, while it was distinctly more common among the orientals. Racial differences in age, height, weight, triceps skinfold thickness, completeness of glucose load ingestion, length of time since last eating, time of day when the test was performed, thiazide usage, educational level, and cigarette smoking history did not explain these findings.