OBJECTIVES: Recently, coinsurance has become a common cost-sharing feature of benefit plans. The objectives were to study annual trends in out-of-pocket expenditures (OOP) and compare OOP for patients with chronic conditions with and without benefit plans requiring coinsurance. METHODS: Inpatient, outpatient, and prescription utilization and expenditure data from 2002 through 2004 were obtained from the Medstat commercial claims database. Benefit design information was available for 5.9 million adults with claims. Adult patients eligible for both medical and drug coverage with at least one inpatient or two outpatient diagnoses of chronic kidney disease (CKD), multiple sclerosis (MS), rheumatoid arthritis (RA), or diabetes were selected. Total OOP were calculated by summing copayments, coinsurance and deductibles for all pharmacy and medical claims. Annualized OOP for patients whose benefit plans required medical and/or pharmacy coinsurance inside the network were compared with patients whose plans did not have any coinsurance requirements. RESULTS: Total of 32,513 patients with no-coinsurance and 293,907 with co-insurance met all other selection criteria. Average OOP for patients in co-insurance plans were two to three times greater than those for patients not in co-insurance plans. During the period from 2002 – 2004, average OOP for the sum of outpatient services, drugs and inpatient visits was higher for patients in plans with in-network coinsurance compared with patients in plans with no coinsurance: CKD ($2022 vs. $759, P <0.0001), MS ($1856 vs. $692, P <0.0001), RA ($1586 vs. $616, P <0.0001), diabetes ($1384 vs. $532, P <0.0001). CONCLUSION: Out-of-pocket expenses are much higher for patients with insurance plans requiring payment of coinsurance. The trend toward coinsurance requirements may limit health care affordability for many patients with serious and chronic conditions.