Optimal treatment for children younger than 5 years of age with insulin-dependent diabetes mellitus is not well defined. Nineteen young children with this disease were treated with a program in which frequent home blood-glucose monitoring was used as the basis for an educational program emphasizing parental adjustment of insulin in response to current glucose levels and anticipated diet and exercise. Eleven children were treated from diagnosis (group I) and another eight (group D) were referred after less intensive treatment. The mean duration of observation of group I children was 13.6 months (range, six to 24 months). For group D, the mean time between diagnosis and referral was 14.9 months (range, seven to 24 months) and 14.6 months (range, six to 24 months) after referral. Before referral, there were 11 hospitalizations in group D. During the intensified program there were two hospitalizations in group D and one in group I. There were 3.3 episodes of severe hypoglycemia per child per 18 months in group D before referral, 1.7 episodes after referral, and 0.4 episodes in group I. Ten of 14 severe hypoglycemic episodes during intensified treatment occurred when there was no or infrequent home blood-glucose monitoring. Only four episodes seemed to have been unpredictable and unpreventable. Mean glycosylated hemoglobin levels were higher in group D patients when compared with both the duration of insulin-dependent diabetes mellitus and the time of initiation of intensified treatment. Mean daily insulin doses increased progressively in group I patients following diagnosis, and were comparable with those in group D patients at 15 and 18 months' duration of illness. Thus, frequency of hospitalization and severe hypoglycemia can be decreased in young children. Frequent home blood-glucose monitoring is required and extensive educational and psychosocial support is necessary for families to implement this intensive approach. The long-term effects on psychoneurological development need evaluation.