Initial blood sugar levels were related to diabetes mellitus in a 17 year follow-up of the pioneer study in Oxford, Mass. Clinical diabetes was found 5 and 15 times more frequently for initial postprandial blood sugars (Folin-Wu) of 140 to 169 mg and 170 mg/100 ml and higher, respectively, than for those below 140 mg/100 ml. Glycosuria was important only when the blood sugar was above 170 mg/100 ml. A confirmatory repeat blood sugar level ≥140 mg/100 ml proved valuable in defining a high risk group for diabetes. The risk of diabetes in overweight persons was significant only when the blood sugar was above 140 mg/100 ml. The similar weight experience of the participants, regardless of their initial blood sugar level, makes it unlikely for obesity to be causally related to diabetes mellitus. The apparently contradictory results from postprandial and postglucose blood sugars are discussed.