The effect of insulin treatment on the rate of decline of plasma glucose concentration was determined in nine patients with hyperosmolar hyperglycemic nonketosis [HHNK; mean plasma glucose, 999 +/- 59 (+/- SEM) mg/dl] and in six normal subjects rendered hyperglycemic by a combined infusion of somatostatin and glucose (mean plasma glucose, 653 +/- 28 mg/dl). Both the fractional glucose turnover and the half-time of the fall in plasma glucose during low dose (5-10 U/h) insulin treatment were reduced 10-fold (P less than 0.001) in the diabetic patients compared with the hyperglycemic normal subjects. In the hyperosmolar patients, the mean glucose clearance during insulin treatment was only 7% that in the normal subjects (P less than 0.001). The rate of plasma glucose decline in our hyperosmolar patients after hydration and insulin administration was 80 +/- 7 mg/dl X h. This decline is comparable to the results reported in other series, although in striking contrast to the 508 +/- 32 mg/dl X h decline in normal subjects (P less than 0.001). Our findings do not support the clinical impression that HHNK patients are insulin sensitive. We conclude that marked resistance to infused insulin delays the correction of hyperglycemia during treatment of HHNK and suggest that resistance to the normal basal insulin levels encountered in some HHNK patients may contribute in part to the development of the hyperosmolar state.