Epidemiologic data on the frequency of diabetes in the urban Greek population were lacking in Greece. Postpranadial urine samples of 21,410 inhabitants of a subturb of Athens were examined for glycosuria by two different enzymatic methods. It was found that 569 persons, i.e. 3.20% of the whole population aged 10 or more, presented postprandial glycosuria (cases of previously known diabetes were excluded); 417 persons out of these were submitted to an OGTT (50 g) and 135 new cases of diabetes were discovered. Glycosuria was associated with diabetes mostly in the age-groups above 40. Obesity was frequent among the newly discovered diabetes. Two thirds had abnormal blood sugar levels already in the fasting condition. The prevalence of non-diabetic glycosuria was 2.70% in males and 1.60% in females. The frequency of a family history positive for diabetes was found to be approximately twice as high in diabetics compared to subjects without glycosuria.