The aim of the present work was to examine the possibility of improving the metabolic control of ambulatory unstable diabetic patients by continuous subcutaneous infusion of insulin. For this purpose, a simple portable pump infusing insulin at a constant rate of 0.8 or 1.1 U/h was utilized. Pulses of a more concentrated insulin solution were superimposed 30-60 min. before meals. Blood glucose control during conventional insulin therapy and during continuous infusion was compared by measuring the mean blood glucose and the M-value of Schlichtkrull. Blood glucose fluctuations were estimated by the MAGE index of Service and Molnar. Three patients were submitted to continuous subcutaneous infusion of insulin during periods of 1, 5 and 6 weeks. In two cases, the application of this technic considerably improved the metabolic control as evidenced by a remarkable decrease of the three measured parameters. In one patient, continuous infusion of insulin was stopped after one week since it did not lead to an improvement of diabetic equilibrium. This failure was apparently due to emotional intolerance of the new technic. On two occasions, infection at the site of infusion was noted. These results should encourage longer-term studies involving a larger number of patients.