81 Background: Immune checkpoint inhibitors (ICIs) have resulted in a subset of patients experiencing durable responses in solid and liquid malignancies. ICIs can be associated with adverse events, including gastroenterocolitis (GEC), that require immunosuppression and even hospitalization. Currently, no data determine the severity of patients hospitalized for ICI-related GEC. Our primary objective was to identify candidate surrogate endpoints that may predict the severity of ICI-related GEC; our secondary objective was to identify predictors of glucocorticoid (GC) response. Methods: In a retrospective cohort study, we identified melanoma patients who developed ICI-related GEC requiring hospitalization at Massachusetts General Hospital between 6/1/11 and 12/31/17. We extracted clinical, laboratory, radiographic, and pathological data; linear regression was used to estimate trends. Additional subgroup analyses were performed. Results: 69/1842 (3.7%) melanoma patients treated with ICI developed GEC requiring hospitalization (total 98 admissions). Mean age was 64 +/- 13; 42 (61%) were male. Readmission rate was 21/69 (30.4%); 6/21 (28.6%) required multiple readmissions. 90/98 (92%) were confirmed by histopathologic examination. 26/69 (37.7%) responded to GCs; 43/69 (62.3%) required second-line immunosuppression (e.g. TNFi) and/or operative intervention. ECOG PS (at initial ICI administration) was associated with response of GEC to GCs (p = 0.04). Lymphocyte count (p = 0.03), % lymphocyte count (p = 0.02), and LDH (p = 0.004) on admission were independently associated with GEC requiring second-line immunosuppression. Conclusions: Admissions for ICI-related GEC are infrequent, but they are associated with a high readmission rate and the need for second-line immunosuppression. We show that high ECOG PS at time of ICI administration may be a predictor of GC response in ICI-related GEC requiring hospitalization. We also propose that low % lymphocyte count and high LDH may serve as clinical biomarkers of severity in ICI-related GEC requiring hospitalization. Our findings suggest the need for further research and validation of these proposed biomarkers.