3072 Background: Cancer of unknown primary (CUP) has a poor prognosis with a median survival of less than 12 months. SUPER is a prospective cohort study designed to create a national biobank of patients (pts) with no confirmed primary site following diagnostic work-up. Tumor and blood samples undergo mutational profiling for actionable mutations using the 386 gene PeterMac Comprehensive Cancer Panel (CCP) plus CUPGuide, a microarray gene-expression site-of-origin assay. We aimed to determine the clinical impact of CUPGuide and CCP profiling. Methods: 172 pts were enrolled between 2013-2015. Baseline demographics, treatments, investigations and clinico-pathological characteristics were collected over 12 months. Clinicians completed clinical management questionnaires before and after receiving results. Results: Molecular analysis was performed for 124/172 (72.1%) pts with sufficient DNA and/or RNA. CUPGuide was completed for 97/124 (78.2%); primary site predictions were made in 84/97 patients (86.6%). The most common primary site predictions were lung, gastric, ovary and breast. CUPGuide predictions resulted in a change in management in 10/84 (12%) of cases and confirmed current management already commenced by the clinician in 53/84 (63%). Mutation profiling was completed in 103/124 (83.1%) pts with actionable mutations found in 11 pts, 4 of whom received subsequent targeted therapy. Testing was considered to have a clinical impact in 70/120 cases (58%): either resulting in a change in treatment (n = 14), diagnosis of a pathogenic germline finding (n = 8) or a moderate/high confidence tissue of origin prediction (n = 58). There were two deaths prior to the availability of the CUPGuide results and eleven deaths prior to availability of the CCP results. Conclusions: Molecular analysis for CUP pts has clinical impact in the majority of cases. Timeliness of return of results, drug access and insufficient tissue for testing are barriers to greater impact that need to be addressed to improve the care of pts affected by CUP.