Background: Down syndrome (DS) is the primary genetic cause of mental retardation and seizures are present in an estimated 5-13% of cases. One-third of seizures in DS are infantile spasms (IS). Hypsarrythmia (HS) is the cardinal electroencephalogram (EEG) feature of IS and has been found to affect cognition; however, its effect on DS patients is inconclusively reported. This study assesses the correlation of HS with cognitive outcomes in DS using the largest sample size to date. Methods: Retrospective study of medical records of children with DS [0-18yrs] at SickKids Hospital in Toronto, from 1990-2013. Seizure history, EEG findings, comorbities, and pharmacological treatments were identified. Developmental outcomes were also assessed from physician comments on motor, verbal and cognitive abilities. The cognitive outcomes of DS patients with and without HS were compared. Results: 70 [male=40] patients with DS and seizures were included. Among 31 (44.2%) patients with DS and IS, 27 had HS. Chi-square analysis showed a significant difference [P=0.007] in prevalence of severe developmental delay in patients with IS and HS versus all other seizure types. Conclusions: The developmental outcome of patients with Down syndrome appears to worsen when IS and HS had occurred in the first year of life.