We investigated the efficacy of insulin-pump therapy in insulin-dependent diabetics, aged 18 to 69 years, by comparing the metabolic control achieved in 100 patients using this technique with that previously obtained by conventional insulin therapy. Patients were followed during pump therapy for as long as 15 months. Fasting and nonfasting blood glucose levels (mean +/- S.E.M.) decreased from 201 +/- 6 and 213 +/- 6 mg per deciliter (11.2 +/- 0.3 and 11.8 +/- 0.3 mmol per liter), respectively, to 158 +/- 5 and 145 +/- 3 mg per deciliter (8.77 +/- 0.3 and 8.05 +/- 0.2 mmol per liter) after one month of pump therapy (P less than 0.001). Ninety-three patients had improved blood sugar control; 71 per cent had a mean blood sugar concentration of 150 mg per deciliter (8.3 mmol per liter) or less after six months. Glycosylated hemoglobin values became normal in 44 per cent of 88 patients who had follow-up determinations. In over 500 patient-months there were four episodes of ketoacidosis and five episodes of serious hypoglycemia. Three patients abandoned pump therapy. We conclude that insulin-pump therapy is acceptable to patients and that it can be successfully applied to clinical practice and large-scale research studies.