Abstract Introduction Idiopathic hypersomnia (IH) is a rare central hypersomnolence disorder with no approved treatment, characterized by excessive daytime sleepiness, prolonged sleep time, and sleep inertia. The Idiopathic Hypersomnia Severity Scale (IHSS) is a 14-item, self-reported questionnaire that assesses severity of IH symptoms, including symptoms related to night/inertia (component I) and day/performance (component II). Individual IHSS items measure symptom frequency, intensity, and consequences using 3- or 4-point Likert scales, yielding a total score (range, 0–50), comprising component I (range, 0–16) and component II (range, 0–34). Higher scores indicate worse symptoms. In a recent clinical trial of the efficacy and safety of lower-sodium oxybate (LXB; Xywav™) for the treatment of IH, the IHSS was a key efficacy measure. Methods Eligible participants 18–75 years of age with IH began LXB treatment with an open-label treatment titration and optimization period (OLTTOP; 10–14 weeks), followed by a 2-week stable-dose period (SDP). Participants were randomized to placebo or continued LXB treatment during a 2-week, double-blind, randomized withdrawal period (DBRWP). The IHSS was completed at baseline, during OLTTOP (weeks 1, 4, and 8), and at the end of OLTTOP, SDP, and DBRWP. Change in IHSS total score from SDP to DBRWP was a key secondary endpoint. Results The efficacy population included 115 participants (mean±SD age, 41±14 years; 71% female). At baseline and the end of SDP, respectively, mean±SD IHSS scores were 31.6±8.3 and 15.3±8.5 for total score, 10.3±3.6 and 5.4±2.8 for component I (night/inertia), and 21.2±5.8 and 9.9±6.5 for component II (day/performance). Worsening from SDP to DBRWP was observed in patients randomized to placebo compared with LXB in IHSS total scores (estimated median difference 95% CI, −12.0 −15.0, −8.0; significant P<0.0001), component I scores (LS mean difference 95% CI, −3.9 −4.9, −2.9; nominal P<0.0001), and component II scores (LS mean difference 95% CI, −7.8 −9.6, −5.9; nominal P<0.0001). Results on all individual IHSS items reflected an improvement with LXB treatment over time during OLTTOP, which remained consistent during SDP. Conclusion These results support the efficacy of LXB for the treatment of IH symptoms, as assessed with the IHSS. Support (if any) Jazz Pharmaceuticals