Background and Objectives. Insulin replacement regimens now stress the importance of administering throughout the day insulin doses that are based on flexible food choices and focusing on improved metabolic control. A flexible multiple daily insulin (FMDI) regimen (premeal lispro plus bedtime glargine) results in lower hemoglobin A(1c) (HbA(1c)) levels and fewer hypoglycemic episodes than does a multiple daily insulin (MDI) regimen among school-aged children and adolescents with type 1 diabetes mellitus (DM). The purpose of this study was to determine the feasibility of FMDI therapy for a group of preschool-aged children with type 1 DM who were transitioned from MDI therapy ( premeal lispro plus ultralente insulin twice per day), by comparing BMI, total daily insulin requirements, HbA(1c) levels, and episodes of severe hypoglycemia. Research Design and Methods. Data were collected over a 2-year period, during quarterly DM clinic visits, from 35 patients ( 17 female patients and 18 male patients, 4.8 +/- 1.0 years of age) who had received MDI insulin therapy for >= 1 year before being transitioned to a FMDI regimen. Results. Although there was no significant change in BMI with FMDI therapy (17.1 +/- 1.8 kg/m(2) vs 17.0 +/- 1.7 kg/m(2)), 43% of patients (6 female subjects and 9 male subjects) were overweight ( BMI of > 85th percentile for age) both before and after treatment. The total daily insulin requirement (0.67 +/- 0.13 U/kg per day vs 0.78 +/- 0.14 U/kg per day) and bolus/basal insulin ratio (1.1 +/- 0.4 vs 1.9 +/- 0.6) were significantly increased and overall glycemic control was improved after transition to FMDI therapy ( HbA(1c) levels: 8.8 +/- 0.9% vs 8.3 +/- 0.8%). However, HbA(1c) levels improved only among normal-weight subjects (9.0 +/- 1.0% vs 8.3 +/- 1.0%) and not among overweight subjects (8.7 +/- 0.7% vs 8.4 +/- 0.6%) after FMDI therapy. The overall rate of severe hypoglycemia was significantly decreased with the FMDI regimen (25.5 events per 100 patient-years vs 10.6 events per 100 patient-years) but again only for normal-weight children (29.7 events per 100 patient-years vs 7.4 events per 100 patient-years). Conclusions. The use of FMDI therapy with glargine among preschool-aged children with type 1 DM was associated with improved overall glycemic control and decreased frequency of severe hypoglycemia. Although our study did not have a control group, these findings suggest that FMDI regimens may be a feasible therapeutic alternative to MDI treatment for preschool-aged children with type 1 DM. However, excess body weight status appeared to preclude a desirable therapeutic response in this group of patients.