Background: Despite advances in understanding the molecular pathways of HCC, therapeutic options are limited and patient survival is dismal. IO is a promising HCC treatment. There are currently no indicators to identify which patients (pts) will have a prolonged response. Methods: In this single-institutional retrospective analysis, pts received one of five IO containing regimens with nivolumab, pembrolizumab, atezolizumab plus bevacizumab, durvalumab or cemiplimab until disease progression (PD) or unacceptable toxicity. Relevant factors including: stage, viral etiology, vascular invasion (VI), tumor thrombus (TT), multifocal disease, toxicity grade, steroid use for IO mediated toxicities and derived Neutrophil-to Lymphocyte ratio (dNLR), were correlated to clinical outcome: progression free survival (PFS), overall survival (OS), response rate (RR), using Pearson’s chi-square test or student's t-test . Responses were assessed using RECIST v 1.1 criteria for stable disease (SD), partial response (PR) and PD were correlated with best response and PFS. OS was calculated by the Kaplan-Meier method. Results: Cohort demographics (n = 76) were: 72% male; 38% African American, 30% Caucasian and 16% Asian; 29% of pts had HBV, 41% had HCV, 1% had both HBV/HCV and 13% had no viral etiology (n = 64). The majority of pts were stage III (43%) or IV (38%). At the start of IO, 32% had VI, 32% had TT and 80% had multifocal or metastatic disease. 65% of pts experienced IO toxicity, with 24.3% at grade 3 or higher, and 34% requiring steroids. Best response to IO was SD in 65.7% of pts, PR in 25.7% and PD in 8.6%. Median OS was 13m (95% CI 7.9-18.1) from the start of IO and median PFS (n = 65) was 14m (95% CI 6.8-21.2). Median OS and PFS were significantly improved in pts with PR compared to PD (45 vs 8m, p < 0.0005, PFS 15 vs 3m p = 0.007). Both OS and PFS showed benefits for SD of ≥2 months compared to those with PD (11 vs 8m, p < .0005, PFS 5 vs 3m p = .007). VI, TT, stage, viral etiology, toxicity grade or dNLR did not correlate with OS, PFS and RR, however need of steroid treatment trended toward worse outcome. Conclusions: PR and SD are independent predictors for prolonged PFS and OS in HCC pts receiving IO therapy. Absence of steroid use for toxicity trended toward improved IO response.