BACKGROUND: The Movement Disorders Society Unified Parkinson Disease Rating Scale (MDS-UPDRS), Part III, is the gold standard for assessing motor symptoms in Parkinson's disease (PD). However, motor symptoms fluctuate significantly from day to day, potentially limiting the sensitivity of this scale for trials with short duration and crossover designs. This study investigated whether day-to-day variability in motor symptoms exceeds the minimal clinically important difference (MCID) in the MDS-UPDRS, Part III. METHODS: Twenty PD participants (Hoehn & Yahr stages 1.5-3) underwent 10 weekly off-medication assessments by one assessor on the same morning. Several determinants of day-to-day variability were explored. RESULTS: Symptom variability often exceeded the MCID for worsening and improvement. Current mental stress and fatigue did not correlate with worse scores, nor did physical activity and sleep quality in the previous week. CONCLUSIONS: These findings suggest that day-to-day symptom variability impacts MDS-UPDRS scores in smaller and shorter-duration trials of symptomatic interventions. Continuous monitoring using wearable sensors may offer more accurate and reliable measures for evaluating PD motor symptoms in clinical studies. © 2025 The Author(s). Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.