OBJECTIVE: One of the newest drug classes used in the treatment of type-2 diabetes is the thiazolidinediones (TZDs). TZDs enhance insulin action in insulin-dependent tissues without stimulating insulin secretion. These drugs are used both as monotherapy and in combination with other anti-diabetic pharmacotherapies. Two TZDs are currently on the market, rosiglitazone and pioglitazone. Both of these compounds were approved in the U.S. mid-year 1999. This study examines the patient characteristics, drug use, and resource utilization of individuals treated with a TZD in the first year following approval. METHODS: Patients who received at least one prescription for rosiglitazone or pioglitazone were selected from over 2.3 million employees, retirees, and dependents in the MarketScan® databases. Health plan enrollment data, medical claims, and pharmacy claims were used to construct the final sample. Descriptive information is presented on patient demographics, inpatient, outpatient and prescription drug use. Comparisons between these variables across the different therapy groups (e.g., monotherapy versus combination therapy) were made using appropriate statistical tests. RESULTS: 18,801 patients received at least one prescription for rosiglitazone or pioglitazone during the study period. Mean age is 60.0 years (SD=11.6 years), including 6,510 who were 65 or older. The sample is almost evenly split between males and females (52.3% and 47.7%, respectively). 54.9% (10,324) of the patients had at least three months follow-up available. Of these, 6.3% (655) were hospitalized within three months of TZD initiation. Patients filled other types of diabetes medications after initiating therapy with a TZD - 28.8% (n=5,417) insulin, 52.0% (n=9,776) sulfonylureas, 39.8% (n=7,475) metformin, and 7.1% (n=1,340) other types of diabetes drugs. CONCLUSIONS: Additional TZD-type compounds are being developed. This study provides an early look at the characteristics and subsequent outcomes of patients who are being prescribed this important new type of therapy for diabetes.