Seven patients with juvenile-onset, insulin-dependent diabetes (aged 13 to 32 years) were continuously treated for 12 to 32 weeks while out of the hospital in their usual environment with a portable, battery-powered infusion pump which delivers insulin subcutaneously in basal (between-meal) doses with pulse dose increments before meals. Mean blood glucose levels (237 +/- 28 mg/dl during conventional insulin therapy) fell to 105 +/- 5 mg/dl after four weeks of pump treatment (p less than 0.01) and were maintained at 80 to 104 mg/dl as pump treatment was continued beyond eight weeks. Glycosylated hemoglobin levels (16.0 +/- 1.5 per cent before pump therapy) also fell within two weeks (p less than 0.01) reaching normal values (9.9 +/- 0.3) after eight weeks of pump therapy. Mean plasma cholesterol and triglyceride levels were elevated during conventional therapy and fell to normal after pump treatment. After the first month of pump treatment, only minor adjustments in insulin dose (less than 5 per cent of total daily dose) were made. No episode of mechanical pump failure occurred during the 1,110 patient-days of treatment. Overinsulinization and underinsulinization due to human error were relatively rare (four and six episodes, respectively) and failed to result in symptoms of hypo- or hyperglycemia. All patients performed their usual home, work or school activities during pump treatment. We conclude that normalization or near normalization of blood glucose levels can be achieved with a portable subcutaneous insulin infusion system when continuously used to treat patients with juvenile-onset, insulin-dependent diabetes outside the hospital for three to eight months.