EWO (Epidemiology of Wearing-Off symptoms among the population of Parkinson’s disease (PD) patients on L-DOPA) is a multicentric, non-interventional, epidemiological and exploratory trial, focused on patients with PD who are treated with L-DOPA. The primary objective is the estimation of wearing-off symptoms (based on WOQ-9 questionnaire and assessment by neurologists) among PD patients who are treated with L-DOPA. From September 30, 2007 to June 30, 2008 altogether 563 valid records of PD patients were collected in movement disorders centers (Level A and Level B centers). Wearing-off symptoms were observed in 66.7% of PD patients (neurologists’ assessment) and in 90.6% of PD patients (WOQ-9 questionnaire). The biggest discrepancy was found in PD patients treated with L-DOPA for 0–2 years. The probability of detecting wearing-off by the neurologists strongly correlated with the highest number of positive responses in the WOQ-9 marked by PD patients. There was significant difference in the diagnosis of wearing-off between Level A centers (77.7%) and Level B centers (62.0%). When motor and non-motor symptoms in the WOQ-9 were considered, the difference between neurologists came from the detection of non-motor symptoms (lower in the Level B centers). The neurologists’ assessment of wearing-off symptoms and PD patients’ subjective evaluation of the WOQ-9 is based on the experience of the neurologist and on the detection of the non-motor symptoms of wearing-off. Careful explanation of the WOQ-9 questionnaire and improved awareness of the non-motor symptoms of wearing-off is strongly suggested.