Introduction The care for many patients with diabetes mellitus type 2 in the Netherlands, is contracted by a local care group. The healthcare providers, who collectively shape a care group, provide protocolled diabetes care. Differences exist between care groups in terms of their organizational and financial arrangements. These differences may result in variation in outcomes. The aim of this study is to assess whether variation in healthcare costs, diabetes complications and related hospital admissions on the level of care groups exist. Methods A quantitative cohort study was conducted. Patients who used diabetes medication (more than 180 days of defined daily doses per year) for the first time between the years 2014 and 2019 were included. Data were extracted from health insurance claims between 2014 and 2019. Generalized linear mixed models were used to analyse patient variation in healthcare costs (two and six years follow-up), diabetes-related complications and hospital admission days. Intraclass correlation coefficients were calculated to estimate the amount of variation that was attributable to the care groups. Results A large variation in outcome variables was observed between patients and a small variation between care groups. The intraclass correlation coefficient for long-term costs was 0.4%; for short-term costs between 0.1% and 0.3%; for complications 1% and for hospital days 4%. Discussion A large variation between patients with diabetes mellitus type 2 exists in terms of their healthcare costs and complications. In our study, care groups accounted minimally for this variation. A generalized linear mixed model in combination with year cohorts is a tool to study variations in the long-term outcomes of integrated care initiatives.