Glucose tolerance data from the second National Health and Nutrition Examination Survey were analyzed to determine factors predicting fasting plasma glucose levels and glucose tolerance in a representative US population. Central adiposity (subscapular skinfolds), age, and family history of diabetes were the major predictors of fasting levels and of glucose tolerance. For women, having a diabetic mother was significantly related to fasting glucose and glucose tolerance, but for men the proportion of siblings with diabetes was positively related. Multiple regression analyses, after adjustment for age and subscapular skinfolds, identified white cell count, systolic blood pressure, natural logarithm total iron binding capacity, and family history variables as being significant predictors of glucose tolerance in both sexes. In addition, several other variables were predictive for men or women, but not for both. These data confirm the importance of the major predictors of glucose tolerance and suggest provocative new associations in the general population.