Glycemia varies widely in patients with diabetic ketoacidosis (DKA), with plasma glucose concentrations between 10 to 50 mmol/L commonly encountered. The mechanism of this glycemic variability is uncertain. Our study examined the differential effects of fasting and dehydration on hyperglycemia induced by withdrawal of insulin in type 1 diabetes. To evaluate the respective roles of dehydration and fasting in the pathogenesis of DKA, 25 subjects with type 1 diabetes were studied during 5 hours of insulin withdrawal before (control) and after either 32 hours of fasting (n = 10) or dehydration of 4.1% +/- 2.0% of baseline body weight (n = 15). Samples were obtained every 30 minutes during insulin withdrawal for substrate and counterregulatory hormone levels and rates of glucose production and disposal. Fasting resulted in reduced plasma glucose concentrations compared with the control study, while dehydration resulted in increased plasma glucose concentrations compared with the control study (P < .001). Glucose production and disposal were decreased during the fasting study and increased during the dehydration study compared with the control study. Glucagon concentrations and rates of development of ketosis and metabolic acidosis were increased during both fasting and dehydration compared with control. These data suggest that fasting and dehydration have differential effects on glycemia during insulin deficiency, with dehydration favoring the development of hyperglycemia and fasting resulting in reduced glucose concentrations. This finding is probably attributable to the differing effect of these conditions on endogenous glucose production, as well as to differences in substrate availability and counterregulatory hormone concentrations. The severity of pre-existing fasting and dehydration likely explains much of the variability in plasma glucose concentrations observed in DKA.