Objective: To validate the Idiopathic Hypersomnia Severity Scale (IIHSS), a self-report measure of hypersomnolence symptoms, consequences, and responsiveness to treatment. Methods: The 14-item IHSS (developed and validated by sleep experts with patients' feedback) was filled in by 218 participants (2.3% missing data). Among the 210 participants who fully completed the IHSS, there were 57 untreated and 43 treated patients with idiopathic hypersomnia (IH) aged 16 years or older, 37 untreated patients with narcolepsy type 1 (NT1), and 73 controls without sleepiness. IHSS psychometric properties, discriminant diagnostic validity, and score changes with treatment were assessed. Results: The IHSS showed good internal consistency and content validity. Factor analysis indicated a 2-component solution with good reliability expressed by satisfactory Cronbach α values. IHSS scores were reproducible without changes in the test-retest evaluation (13 treated and 14 untreated patients). Convergent validity analysis showed that IHSS score was correlated with daytime sleepiness, depressive symptoms, and quality of life in patients with IH. The IHSS score was lower in treated than untreated patients (5-8 unit difference, without ceiling effect). The cutoff value for discriminating between untreated and treated patients was 26/50 (sensitivity 55.8%, specificity 78.9%). IHSS scores were higher in drug-free IH patients than NT1 and controls. The best cutoff value to differentiate between untreated IH patients and controls was 22 (sensitivity 91.1%, specificity 94.5%), and 29 with NT1. Conclusions: The IHSS is a reliable and valid clinical tool for the quantification of IH symptoms and consequences that might be useful for patient identification, follow-up, and management.