Six 17- to 53-month-old diabetic children had marked metabolic instability characterized by chronic hyperglycemia and frequent or severe hypoglycemia with conventional management that included twice daily insulin injections, diet, and home blood glucose monitoring. Because of the metabolic instability, all were given continuous subcutaneous insulin infusions (CSII) via portable externally worn infusion pump. During 6 months of CSII, there was marked improvement: hemoglobin A1 decreased from 192% +/- 8% (SD) to 152% +/- 31% of the normal mean (P less than 0.02), and hypoglycemic episodes decreased in both severity and frequency. CSII was incorporated into the children's treatment with no appreciable adverse psychologic effects or interference with normal activities. CSII, under carefully controlled clinical conditions, may be of benefit in some preschool children with unacceptable metabolic control of diabetes mellitus.