This randomized, double-blind, active-controlled, 2 parallel-group, multicenter study evaluates the efficacy and safety of a weekly subcutaneously injected dose of albiglutide as compared with sitagliptin. Subjects who are renally impaired with a historical diagnosis of type 2 diabetes mellitus and whose glycemia is inadequately controlled on their current regimen of diet and exercise or their antidiabetic therapy of metformin, thiazolidinedione, sulfonylurea, or any combination of these oral antidiabetic medications will be recruited into the study.