PURPOSE: Documenting diabetes treatment patterns and associated costs over time is an important step in gauging the medical and economic impact of current treatment guidelines in a real-world setting. This study was designed to assess changes in medication treatment patterns, health care costs, and comorbidities over a 6-year period after a new diagnosis of type 2 diabetes mellitus (T2DM). This analysis is the first of its kind to observe, over time, a single US cohort of patients newly diagnosed with T2DM. METHODS: This study was a longitudinal assessment of changes in medical services and comorbidities for a single cohort (N = 35,017) of adults newly diagnosed with T2DM in 2006 using claims data from Truven Health Analytics MarketScan(®) databases. Prevalence of diabetes-related comorbidities and utilization/costs of inpatient/outpatient services and medications were analyzed annually for the index (diagnosis) year (Y1) through year 6 (Y6) postindex. Costs were adjusted to 2012 dollars. FINDINGS: From Y1 to Y6, increased prevalence was noted for several T2DM-associated comorbidities: cerebrovascular disease (13%-21%), peripheral vascular disease (3%-10%), nephropathy (3%-13%), and retinopathy (4%-14%). All-cause costs of inpatient and outpatient services and medications were analyzed for the index year (Y1) through Y6 postindex (adjusted to 2012 dollars). Total health care utilization costs (services plus drugs) increased by 33.3% from Y1 ($329.8 million) to Y6 ($439.5 million). Inpatient costs across the entire cohort increased 19.3% from Y1 ($49.8 million; $1421/patient) to Y6 ($59.4 million; $1695/patient) but increased 46.6% among utilizers, despite a decline in inpatient utilizers (7.3% to 5.9% of patients). The percentage of outpatient services utilizers remained stable (Y1, 98.2%; Y6, 97.2%), but total visits increased by 9.1%. Costs of outpatient services increased by 32.5%, from $145 million (Y1) to $192 million (Y6). Total drug costs increased from $101.5 million (Y1) to $114.7 million (Y6) but accounted for a smaller percentage of all health care costs in Y6 versus Y1 (26.1% vs 30.7%). Antidiabetes drugs accounted for a small percentage of overall costs in both Y1 (3.6%) and Y6 (5.3%). IMPLICATIONS: Overall, we found evidence of increasing comorbidities paralleled by large increases in costs for medical services but less for prescriptions. These findings confirm a need for aggressive diabetes management to slow disease progression and minimize comorbidity and economic burdens of the disease.