The project is a multi-center, prospective, randomized, double-blind, parallel, placebo controlled clinical trial evaluating the efficacy of sitagliptin in suppressing glucagon release in subjects with type 1 diabetes over a 16 week treatment period. The purpose of this study is to determine if sitagliptin (Januvia™) can decrease the paradoxical post-prandial glucagon release and can result in improved glucose control, and thus decrease A1c and improve MBG values as determined by continuous glucose monitoring (CGM) in adult patients with type 1 diabetes. This study will enroll a maximum of 140 patients with A1c values between 7.5-10% who will be randomized in a 1:1 fashion to either the study drug or placebo. The patients will attend 6 clinic visits and have 4 phone call visits. Laboratory analysis will be performed at various time points assessing glucagon, A1c, C-peptide, glucagon-like-peptide-1 (GLP-1), glucose-dependent insulinotropic peptide (GIP), and insulin levels. The CGM measurements will be obtained on all patients for one week during the run-in period (baseline), after start of study medication week 1, and one week prior to visits at 4, 12, and 16 weeks to assess for MBG values, time spent in hyperglycemic and hypoglycemic excursions, and other indices of glucose variability.