Combined insulin and sulfonylurea therapy for type 2 diabetes may improve the effectiveness of a single injection of insulin, thereby postponing the need for multiple injections. This concept was tested in 21 obese subjects imperfectly controlled by 20 mg of glyburide daily in a double masked, placebo-controlled, parallel design, 16-week protocol. Premixed 70% NPH/30% Regular insulin was taken before supper, and the dosage was adjusted weekly by an algorithm seeking nearly normal fasting glycemia. Eleven subjects using insulin plus 10 mg glyburide before breakfast had lower mean fasting glucose at 10-16 weeks than 10 subjects using insulin with placebo (mean +/- SEM; 5.9 +/- 0.3 versus 7.5 +/- 0.7 mmol/L; p less than 0.05), and had a greater decrement of glycosylated hemoglobin from baseline values (1.3 +/- 0.1 versus 0.8 +/- 0.2% A1, p less than 0.05). After 16 weeks the combined therapy group used half as much insulin as the insulin-only group (50 +/- 5 versus 101 +/- 13 units/d; p less than 0.01). Fasting serum free insulin values increased 58% from baseline after insulin therapy in the insulin-only group (p less than 0.05) but did not increase with combined therapy. Weight gain was similar in the two groups. These data support this form of combined therapy as one option for treating obese persons with type 2 diabetes no longer responsive to oral therapy alone.