OBJECTIVE The aim of this study was to evaluate effects of adjunctive cannabidiol on cognition, and quality of life in children with Lennox-Gastaut syndrome METHODS Forty-one patients who received cannabidiol between March 2019 and August 2019 at Severance Children’s Hospital were included. Cannabidiol was initiated at 5mg/kg/day and titrated to a maximum dose of 10mg/kg/day. Adjunctive cannabidiol was administered over a 2-week titration period and a 13-week maintenance period. Seizure, cognition and quality of life were assessed after 3 months. Study endpoints included change from baseline in seizure frequency, social maturity scale (SMS), Vineland adaptive behavioral scales (Vineland-III), child behavior check list (CBCL), and quality of life (QOL) scales. RESULTS A total of 19 patients completed a 3-month treatment of cannabidiol. Seventeen patients completed follow-up tests. Of 17, 10 were male. Mean age was 6.5 ± 3.9 years. Of 17, 11 (65%) of the patients had daily seizures. Tonic seizure was the most common seizure type (65%). Others had myoclonus (18%) or focal seizures (18%). After 3 months, five (29%) patients experienced 50% or more seizure reduction. Of five, three (18%) patients achieved seizure freedom. Overall QOL improved from baseline after treatment ( 91 (range 62 -149) vs. 118 (range 88 ? 181), p=0.003). QOL regarding physical function, cognition, and general health did not change, but QOL regarding behavior issues improved after treatment from 25.2 ± 16.0 to 30.0 ± 12.7 (p = 0.009). Social age on SMS improved from 2.3 (range 0.8 ? 16) to 2.5 (range 0.8 ? 16.3) years (p = 0.0006) after treatment. Total Vineland score did not change from baseline after treatment (33 (range 26 ? 96) vs. 34 (range 25 - 110), p = 0.2). Treatment did not affect CBCL scores regarding behavioral problems including affective problems, anxiety, pervasive developmental problems, attention, oppositional defiant, and conduct problems. Conclusion These findings suggest that cannabidiol is an effective treatment option for patients with Lennox-Gastaut syndrome. Adjunctive 3-month cannabidiol treatment did not have any significant overall effects on cognition, and improved QOL.