Three “brittle” diabetic patients were given constant subcutaneous insulin infusion with a portable battery-driven pump, and their plasma glucose and hemoglobin A1 were measured at frequent intervals during inpatient or outpatient periods. Mean plasma glucose decreased significantly in all inhospital patients and remained significantly less before pump therapy in two of three as outpatients during the 8–12 wk of follow-ups; however, complete normalization of glucose metabolism was not accomplished in any. All three demonstrated a progressive decline in hemoglobin A1 levels to normal or near-normal values. The need for long-term studies of open loop infusion systems in a research setting before their adaptation to routine care is emphasized.