To improve plasma glucose control, we administered insulin via the subcutaneous route in seven ambulatory patients with juvenile diabetes (12 to 17 years of age), using a portable infusion pump at a basal rate with pulse-dose increments before meals. After two to four days, the mean plasma glucose (+/- 1 S.E.) of 94 +/- 5 mg per deciliter was markedly lower than when insulin was given by conventional methods in the patients' usual dose (243 +/- 28, P less than 0.01) or in a total dose equivalent to that administered with the pump (150 +/- 15, P less than 0.01). Maximal fluctuations in plasma glucose were also 50 to 150 mg per deciliter below those observed with conventional treatment (P less than 0.001). Glycosuria was eliminated in six of seven patients during pump treatment. None of the subjects had hypoglycemia. These results demonstrate that plasma glucose can be lowered to normal in ambulatory patients with brittle juvenile diabetes using a portable, subcutaneous insulin infusion system for two to four days. The feasibility and value of the long-term application of this technic need exploration.