Background:; Warfarin is efficacious for reducing stroke risk among patients with nonvalvular atrial fibrillation (NVAF). However, the efficacy and safety of warfarin are influenced by its time in therapeutic range (TTR).; Objective:; To assess differences in healthcare resource utilization and costs among NVAF patients with low (<60%) and high (60%) warfarin TTRs in an integrated delivery network (IDN) setting.; Methods:; Patients with NVAF were identified from an electronic medical record database. Patients were required to have 6 international normalized prothrombin time ratio (INR) tests. NVAF patients were grouped into two cohorts: those with warfarin TTR <60% (low TTR) and those with warfarin TTR 60% (high TTR). Healthcare resource utilization and costs were evaluated during a 12 month follow-up period. Multivariable regressions were used to assess the impact of different warfarin TTRs on healthcare costs.; Results:; Among the study population, greater than half (54%, n=1595) had a low TTR, and 46% (n=1356) had a high TTR. Total all-cause healthcare resource utilization was higher among patients in the low TTR cohort vs. the high TTR cohort (number of encounters: 70.2 vs. 56.1, p<0.001). After adjusting for patient characteristics, total all-cause healthcare costs and stroke-related healthcare costs were $2398 (p<0.001) and $687 (p=0.02) higher, respectively, for patients in the low TTR cohort vs. the high TTR cohort.; Limitations:; In this retrospective study, we were only able to evaluate the association and not the causality between healthcare resource utilization and costs with the different warfarin TTRs.; Conclusion:; Many warfarin-treated NVAF patients have a low warfarin TTR. NVAF patients with low vs. patients with high warfarin TTR used healthcare resources to a greater extent, which was reflected in higher healthcare costs.