OBJECTIVE To investigate the effect of 4 weeks of treatment with liraglutide on insulin dose and glycemic control in type 1 diabetic patients with and without residual ß-cell function. RESEARCH DESIGN AND METHODS Ten type 1 diabetic patients with residual ß-cell function (C-peptide positive) and 19 without (C-peptide negative) were studied. All C-peptide-positive patients were treated with liraglutide plus insulin, whereas C-peptide-negative patients were randomly assigned to liraglutide plus insulin or insulin monotherapy. Continuous glucose monitoring with identical food intake and physical activity was performed before (week 0) and during (week 4) treatment. Differences in insulin dose; HbA(1c); time spent with blood glucose <3.9, >10, and 3.9-9.9 mmol/L; and body weight were evaluated. RESULTS Insulin dose decreased from 0.50 ± 0.06 to 0.31 ± 0.08 units/kg per day (P < 0.001) in C-peptide-positive patients and from 0.72 ± 0.08 to 0.59 ± 0.06 units/kg per day (P < 0.01) in C-peptide-negative patients treated with liraglutide but did not change with insulin monotherapy. HbA(1c) decreased in both liraglutide-treated groups. The percent reduction in daily insulin dose was positively correlated with ß-cell function at baseline, and two patients discontinued insulin treatment. In C-peptide-positive patients, time spent with blood glucose <3.9 mmol/L decreased from 3.0 to 1.0 h (P = 0.03). A total of 18 of 19 patients treated with liraglutide lost weight during treatment (mean [range] -2.3 ± 0.3 kg [-0.5 to -5.1]; P < 0.001). Transient gastrointestinal adverse effects occurred in almost all patients treated with liraglutide. CONCLUSIONS Treatment with liraglutide in type 1 diabetic patients reduces insulin dose with improved or unaltered glycemic control.