Background: Understanding the different patterns of adherence to istradefylline treatment is essential to identifying Parkinson's disease (PD) patients who might benefit from targeted interventions., Objectives: This descriptive study aimed to identify longitudinal istradefylline adherence patterns and to characterize factors associated with them., Methods: We identified PD patients aged 21-99 years who initiated istradefylline treatment in a Japanese hospital administrative database. Group-based trajectory modeling was used to model the monthly proportion of days covered over time to identify distinct 360-day adherence patterns. Factors associated with each adherence pattern were assessed using univariable multinomial logistic regression models., Results: Of 2088 eligible PD patients, 4 distinct adherence groups were identified: consistently high adherence (56.8%); rapidly declining adherence (25.8%); gradually declining adherence (8.5%); and gradually declining and then recovering adherence (9.0%). Compared to the consistently high adherence group, the other groups had the following characteristics associated with a likelihood of lower adherence: the rapidly declining adherence group received fewer dopamine agonists (63.8% vs. 69.4%), monoamine oxidase B (MAO-B) inhibitors (26.8% vs. 31.6%), and catechol-O-methyl transferase inhibitors (31.6% vs. 37.0%) and had a higher prevalence of anxiety/mood disorders (29.9% vs. 24.6%); the gradually declining adherence group received fewer MAO-B inhibitors (22.5% vs. 31.6%) and amantadine (8.4% vs. 16.1%) and had a higher prevalence of mild cognitive impairment/dementia (27.0% vs. 18.8%); and the declining and then recovering adherence group had a higher prevalence of anxiety/mood disorders (34.2% vs. 24.6%)., Conclusions: Clinicians should be aware of the heterogeneous patterns of adherence to istradefylline., Competing Interests: Declaration of competing interest T.F. has been employed by the Department of Digital Health and Epidemiology with support from Eisai Co., Ltd. and Kyowa Kirin Co., Ltd.; and has received consulting fees from Real World Data Co., Ltd. and honoraria from EPS Corporation and Research Institute of Healthcare Data Science (RIHDS). E.N. has received consulting fees from Kyowa Kirin Co., Ltd., and honoraria from AbbVie GK, Daiichi Sankyo Co., Ltd., EA Pharma Co., Ltd., Eisai Co., Ltd., Kyowa Kirin Co., Ltd., Ono Pharmaceutical Co., Ltd., Sumitomo Pharma Co., Ltd., and Takeda Pharmaceutical Co., Ltd. H.S., K.S., S.I., and S.A. have been employed by Kyowa Kirin Co., Ltd. S.Y. has no conflicts of interest. S.T.-M. has been employed by the Department of Digital Health and Epidemiology with support from Eisai Co., Ltd. and Kyowa Kirin Co., Ltd.; and has received consulting fees from Real World Data Co., Ltd. and honoraria from RIHDS. K.M. has been employed by the Department of Digital Health and Epidemiology with support from Eisai Co., Ltd. and Kyowa Kirin Co., Ltd. R.T. has received consulting fees from Kyowa Kirin Co., Ltd., and grants from Eisai Co., Ltd., Kyowa Kirin Co., Ltd., Sumitomo Pharma Co., Ltd., honoraria from AbbVie Inc., Eisai Co., Ltd., Kyowa Kirin Co., Ltd., Ono Pharmaceutical Co., Ltd., Sumitomo Pharma Co., Ltd., and Takeda Pharmaceutical Co., Ltd., and subcontracting fees (trial cases) from Sanofi K.K., Takeda Pharmaceutical Co., Ltd., and Teijin Pharma Ltd. K.K. has received consulting fees from Advanced Medical Care Inc., JMDC Inc., LEBER Inc., and Shin Nippon Biomedical Laboratories Ltd., executive compensation from Cancer Intelligence Care Systems, Inc., honoraria from Chugai Pharmaceutical Co., Ltd., Kaken Pharmaceutical Co., Ltd., and Pharma Business Academy, and grants from Eisai Co., Ltd., Kyowa Kirin Co., Ltd., OMRON Corporation, and Toppan Inc., (Copyright © 2023. Published by Elsevier B.V.)