Objective Mean differences in HbA(1c) across centers are well established, but less well understood. The aim was to assess whether differences in patient case-mix can explain the variation in mean HbA(1c) between pediatric diabetes centers in Denmark. The association between HbA(1c), frequency of blood glucose monitoring (BGM), treatment modality, and center visits was investigated. Research Design and Methods This longitudinal nationwide study included 3866 Danish children with type 1 diabetes from 2013 to 2017 (n = 12,708 child-year observations) from 16 different pediatric diabetes centers. Mean HbA(1c), proportion of children reaching HbA(1c) treatment target (HbA(1c)