In North America, gestational diabetes mellitus (GDM) is diagnosed from a 100-g oral glucose tolerance test (OGTT) with criteria proposed by the National Diabetes Data Group (NDDG). These criteria were derived in the 1950s from an unrepresentative sample of women tested predominantly in the latter stages of pregnancy. The original studies did not check for reproducibility of OGTT results. Measurements were made with the Somogyi-Nelson whole-blood glucose technique, and test translation errors are present in the threshold values proposed for modern plasma glucose oxidase methods. Whereas GDM is now diagnosed with a view to adverse m ternal-fetal outcomes, the criteria were chosen to reflect maternal risk of developing glucose intolerance as shown by a 75-g OGTT in the nonpregnant state over the ensuing 8 yr. For that outcome, the positive predictive value of the criteria was only 36.1%, and this is a marked overestimate, because the study cohort was a highly selected group with an increased incidence of glucose intolerance both during and after pregnancy. The criteria are also conceptually flawed in that they impose a dichotomous definition of normal and abnormal on gestational glucose tolerance, when the risk of adverse maternal-fetal outcomes and later diabetes mellitus should logically be graded upward with higher values on the gestational OGTT and with the degree of fasting hyperglycemia. Although NDDG criteria merit continued use for lack of a better alternative, new diagnostic criteria for GDM should be derived and validated.