The sensitivity and specificity of each of five screening tests were estimated in each of three to ten countries by testing subjects drawn from the general populations of adults over thirty-four years of age. This permitted comparisons among countries and among the different tests (fasting, postprandial, and postglucose urine tests, and fasting and postprandial blood glucose values). Sensitivity and specificity of each test varied widely among populations. For example, the sensitivity of the two-hour urine glucose ranged from 17 per cent in Nicaragua to 100 per cent in East Pakistan. Apparently specificity and sensitivity of such tests are influenced by many factors including both the circumstances under which the tests are performed and the characteristics of the population tested. It is, therefore, not possible to predict prevalence rates reliably by extrapolating from the results of screening tests. However, we believe the data for specific populations on the sensitivity and specificity of various tests will provide a rough guide in predicting the cost-effectiveness of alternative approaches to case detection in those particular countries. For instance, these results suggest that roughly 56 per cent of the occult diabetics in Costa Rica in this age group would be detected by a two-hour urine glucose, but only about 41 per cent of those in whom this test was positive would prove to have diabetes. Even modest changes of criteria in defining either "diabetes" or "abnormality" of the screening results produced marked changes in rates of sensitivity and specificity. With few exceptions, tests which were more sensitive were, comparably, less specific, and the reverse was also true. Rates of "diabetes" were markedly influenced by modest changes in diagnostic criteria.