Background: Chronic myeloid leukemia (CML) treatment improved considerably after introduction of oral tyrosine kinase inhibitors (TKI). As a result, the number of patients living with CML may reach 250,000 by 2040. We track changes in TKI treatment adherence since 2001 and provide an early assessment of treatment costs following the availability of second-generation TKIs and generic imatinib. Methods: A retrospective cohort from the OptumLabs Data Warehouse, which includes claims data for privately insured and Medicare Advantage (MA) enrollees in a large private U.S. health plan with medical and pharmacy benefits, was used. Patients with CML initiated TKI treatment between May 2001 and October 2016 and were continuously enrolled in the health plan 6 months prior through 12 months following TKI start. Adherence was defined by medication possession ratio (MPR1=total days’ supply of imatinib in 1st year divided by 365, 1=perfect adherence). Total health care costs include medical and prescription medication benefits. MPR1 was modeled using ordinary least squares regression. The association between MPR1 and healthcare costs was estimated using a generalized linear model specified with a gamma error distribution and a log link. Results: We identified 1,793 eligible patients. First-line TKI has changed over time (dasatinib and nilotinib represent 45% of all 2016 starts; imatinib 55%). From 2001 to 2016, adherence increased (Table 1). MPR1 was higher in men and increased with age until age ∼62 after which it declined. MPR1 was lower for patients with more comorbid conditions prior to treatment. Overall, MPR1 was inversely associated with total health care costs (medical and pharmacy) among privately insured (P<.001) but not MA enrollees. The net impact of MPR1 on total healthcare costs diminished over time (P<.001) where a 10% point decrease in MPR1 was associated with 12% and 4% lower total costs, prior to and following availability of 2nd generation TKIs, respectively. When examining medical costs only, MPR1 was inversely associated with medical costs for both privately insured (P<.001) and MA enrollees (P=.016). Conclusions: We found that adherence to TKI treatment increased over time. While imatinib is still used more frequently than other TKIs as first-line therapy, second-generation TKIs are becoming increasingly used as first-line agents. Possible cost-offsets are decreasing over time but it may be too early to formally evaluate the impact of generic imatinib.