Background: Anti-PD-1 immunotherapy has revolutionized metastatic melanoma treatment, as first-line monotherapy or in combination with Ipilimumab. Up to 40% of patients will progress within 3 months, with limited evidence on who derives benefit from immunotherapy. We report clinical and pathological predictive and prognostic factors from a multi-institutional cohort. Methods: Patients between 2014-2017 treated with Nivolumab and Pembrolizumab were identified from a provincial pharmacy database in Alberta, Canada. All patients had unresectable stage III or IV melanoma. Patient characteristics, investigations, treatment and clinical outcomes were obtained from electronic medical records. We utilized Cox regression and Kaplan-Meier methods to analyze progression free survival (PFS) and overall survival (OS). Results: 143 patients with either cutaneous (115) or primary unknown (28) melanoma were identified. Immunotherapy was median second line treatment and patients received a median of 7 doses. The median age was 64, and 144 (80%) were either ECOG 0 or 1 at treatment initiation. The overall response rate was 33%, with median follow up of 25 months. Ulcerated primary tumors had a lower mOS of 30 months vs. 49 months (p=0.042). Other pathologic factors (including Breslow Depth, tumor infiltrating lymphocytes, mitosis) were not associated with PFS or OS. Clinical factors associated with worsened mPFS and mOS were liver metastases, >3 sites of disease, and any visceral disease. Elevated LDH, platelets, neutrophils, and lower hemoglobin, lymphocytes, and a neutrophil/lymphocyte ratio were associated with worse mPFS and mOS. We identified 4 prognostic subgroups using LDH and number of visceral sites (Table) which was statistically significant for mPFS and mOS. Conclusions: Ulcerated primary tumors, liver metastasis, and more sites of disease had worse mPFS and mOS. We also identified 4 novel prognostic subgroups strongly associated with survival outcomes.