<h3>Objective:</h3> To assess prognostic value of hypsarrhythmia (Hyp) <h3>Background:</h3> There is low interrater reliability of diagnosis of Hyp and no data on its prognostic value related to epileptic spasms (ES). We propose practical criteria to diagnose Hyp and evaluate its prognostic value. <h3>Design/Methods:</h3> VEEG of 34 patients with ES were reviewed pre-vigabatrin therapy (VGNT) (used as 1st-line), and 2–8 weeks (median 4) post-VGNT. All patients had awake and non-REM EEG. TSC diagnosis was excluded. Hyp was diagnosed using the proposed criteria (figure not included). Seizure-freedom w/wo ASM was assessed during an average F/U period of 39.2 months (2–104). <h3>Results:</h3> ES was diagnosed at average age of 9.6 m (2–36). 20/43 (59%) had Hyp, while 14/34 did not. The primary reason for (−) Hyp was due to amplitude < 300 uV (13/14 patients). The major cause of ES <i>with Hyp</i> was genetic with normal brain MRI (8/20) and idiopathic (7/20), while the major cause of ES <i>without</i> Hyp was abnormal cortex (13/14: p<0.01). More patients <i>with</i> Hyp had ES resolution than those <i>without</i> Hyp (13/20 vs 4/14: p<0.05). There was a higher chance of seizure-freedom in patients <i>with</i> Hyp (8/20 vs 1/14: p<0.05). All 7 patients with idiopathic cause had Hyp and attained resolution of Hyp and ES after VGNT, and 5/7 were seizure-free. All 3 patients with Trisomy-21 had Hyp, and attained ES resolution and seizure-freedom (p<0.05). <h3>Conclusions:</h3> Patients <i>with Hyp</i> were more likely to have-idiopathic/genetic cause of ES; ES resolution; a higher chance of seizure-freedom. Patients <i>without Hyp</i> were more likely to have abnormal cortex. Hyp with ES on initial EEG predicts - likely cause of ES, response to treatment, and chance of seizure-freedom - even <i>before any</i> further diagnostic tests. Using the proposed criteria, the most frequent reason for (−) Hyp was due to lack of amplitude of 300μV. <b>Disclosure:</b> Dr. Okahashi has nothing to disclose. Dr. Nemade has nothing to disclose. Mrs. Brown has nothing to disclose. Dr. Park has nothing to disclose.