Introduction: This study examines factors associated with erythropoiesis-stimulating agent (ESA) hyporesponsiveness, the duration of ESA hyporesponsiveness, the frequency of new episodes, and variation across countries. Methods: We used international Dialysis Outcomes and Practice Patterns Study data from 2015 to 2018 (N = 26,656) to investigate changes in ESA Resistance Index (ERI), calculated as epoetin dose divided by [hemoglobin × body weight] in patients on hemodialysis. We illustrated the proportion of patients who moved to other ERI quintiles over 12 months, and we studied the incidence and duration of ESA resistance. We examined case-mix factors associated with quintiles of ERI. Results: Most patients migrated out of their original ERI quintile within 4 months. Only 22% of patients in the top quintile of ERI at baseline (4.4% of all patients) remained in the top quintile during all 12 months of follow-up. A total of 42% of patients manifested an upper-quintile ERI during at least 1 month. Median duration of a new episode of ESA resistance was 2 months. Catheter hemoaccess, elevated C-reactive protein, lower transferrin saturation, lower serum albumin concentration, and recent hospitalization occurred more frequently among patients in the highest ERI quintile at baseline. ERI values were highest in the USA, Italy, and Mideastern nations and lowest in Russia and Japan. Discussion/Conclusion: It is a misconception to envision a sizable, fixed segment of the population with permanent resistance to ESA – resistance fluctuates frequently. The implications of these findings for prescription of ESAs and of hypoxia-inducible factor-prolyl hydroxylase inhibitors are discussed.