Apathy is a behavioural disturbance occurring alone or in concomitance with depression in Parkinson's disease (PD). Here we present a validation study for the self-report version of the Apathy Evaluation Scale (AES-S), carried out in a sample of 60 non-demented, non-depressed untreated, drug-naïve, de novo PD patients; 20 patients of the sample (33.3%) were classified as apathetic according to current clinical criteria. All enrolled patients completed the AES-S and a neurological and cognitive assessment. Mean AES-S score was 34.43. AES-S did not show floor or ceiling effect. Cronbach's alpha was 0.872. Principal component analysis revealed three factors: the first (34.4% of the variance) represented constitutive aspects of the construct of apathy; the second (8.5% of the variance) represented a social dimension; the third factor (7.9% of the variance) represented a dimension related to insight. With respect to clinical criteria for apathy considered as the gold standard, receiver operating characteristics curve analysis showed that a cut-off of 36/37 has the maximum discrimination power. High sensitivity and negative predictive values were obtained with cut-off scores of 33/34 or lower; high specificity and positive predictive values were obtained with cut-off scores of 38/39 or higher. AES-S score correlated with scores on frontal tasks, but not on Beck Depression Inventory, Unified Parkinson's Disease Rating Scale, Hoehn and Yahr scale. The AES-S is a reliable and valid questionnaire for detecting apathy in PD. For screening purposes a 33/34 cut-off score is indicated, but a 38/39 cut-off score is necessary when a high specificity is desired.