Glucose tolerance tests and immunoreactive insulin responses were analyzed in 203 subjects with abnormal glucose tolerance and in 298 nondiabetics. All subjects were grouped as obese (greater than 115 per cent of ideal body, weight) or lean. Those with an abnormal glucose tolerance test were further subdivided: (1) Normal fasting glucose and one or more abnormal subsequent values, and (2) elevated fasting glucose and an abnormal tolerance test. The total mean glucose and insulin concentrations, expressed as the areas under the three-hour glucose tolerance curves, were compared in the six subgroups. Hyperinsulinism was found in obese nondiabetics as compared to lean nondiabetics. Mildly impaired glucose tolerance in both obese and non-obese patients was associated with increased insulin levels compared to nondiabetics. Despite increased immunoreactive insulin levels in subjects with impaired glucose tolerance, the concentrations were less than expected for corresponding glucose levels during the early portion of the tolerance test. Decompensated diabetes, manifested by fasting hyperglycemia, was associated with absolute as well as relative deficiency of endogenous insulin. While definite trends in glucose-induced insulin responses could be determined in specific groups, there was marked individual variation.