Background: The Parkinson Disease (PD) Home Diary (HD) is a commonly used clinical outcome measure, but it has not been extensively compared to direct assessments by experienced observers., Objective: Validation of patient-reported HD by investigating the agreement between motor state assessments by patients and observers., Methods: This observational study included patients with PD and motor fluctuations. Observers were physicians or research nurses. Patients completed a screening visit, one day of diary ratings at home, and then two days of ratings on-site during which patients and observers simultaneously judged the participants' motor state., Results: Observers and 40 patients completed 1,288 pairs of half-hourly blinded motor state assessments. There were significant differences between observer and patient ratings ( P < 0.001) and the temporal agreement was poor (Cohen's κ = 0.358). The agreement between patient and observer ratings was 71.1% for observed "On without dyskinesia", 57.3% for observed "Off", and 49.4% for observed "On with dyskinesia". Daily times spent in the three motor states as aggregated diary data showed fair to excellent reliability with intraclass coefficient values ranging from 0.45 to 0.52 for "On" and 0.77 for "Off"., Conclusion: There were significant differences between observer and patient ratings. Patients and observers generally agreed on when the patients was in the "On" state (with or without dyskinesia). Patient ratings on the hour level seem to be influenced by other aspects of the patients' experience than the observed motor state, but assessment of daily time spent in the different motor state provides reasonable reliability., Competing Interests: JT has received funding from the Swedish National Government and County Councils through the ALF agreement, the Swedish Parkinson Foundation, and the Elsa Schmitz Foundation, and has received compensation for consultancies from AbbVie and TransPerfect, as well as royalties from UNI-MED Verlag. ML has received honoraria for presentations from Novartis Pharma and UCB Pharma. FG reports honoraria from AbbVie, BIAL, Merz, and STADA outside the submitted work. GE received honoraria for Advisory Boards, Consultancy and Presentations from AbbVie Pharma, BIAL Pharma, Biogen GmbH, Desitin Pharma, STADA Pharma, Neuroderm Inc. Licher GmbH, UCB Pharma, Zambon Pharma. He has received royalties from Kohlhammer Verlag and Thieme Verlag. ÖD has received funding from Faculty of Arts and Sciences, Linköping University. SI has received honoraria from Karolinska Institutet and Swedish Agency for Participation. She has received research funding from Ribbingska Foundation in Lund, Swedish Research Council; Swedish Research Council for Health, Working Life and Welfare (Forte); Swedish Research Council for Sustainable Development (Formas), NEURO Sweden, Family Kamprad Foundation, LMK Foundation, Lund University, Lund University Innovation System, Helsingborg City Competence Foundation. MN was funded by the Strategic Research Area in neuroscience (MultiPark) at Lund University, Lund, Sweden. AS has received funding from the Deutsche Forschungsgemeinschaft (German Research Association) and the Helmholtz-Association. He has received honoraria for presentations/advisory boards/consultations from Desitin, Global Kinetics, Lobsor Pharmaceuticals, STADA, Bial, RG Gesellschaft, Zambon, and AbbVie. He has received royalties from Kohlhammer Verlag and Elsevier Press. He serves as an editorial board member of Stem Cells and Stem Cells International. PO has received funding from AbbVie, Lund University Medical Faculty, Multipark, the Swedish Parkinson Foundation, Health Care Region Skåne and Åhlens Foundation. He has received honoraria for lectures and expert advice from AbbVie, Bial, Britannia, Ever Pharma, Global Kinetics, Lobsor, Nordic Infucare, Stada, and Zambon. He has received royalties from UNI-MED Verlag. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 Timpka, Löhle, Bremer, Christiansson, Gandor, Ebersbach, Dahlström, Iwarsson, Nilsson, Storch and Odin.)