Background: Neuroendocrine tumors (NET) can present as hepatic metastatic disease without a known primary site. Questions regarding the optimal surgical management in such cases remain unanswered. The aim of this study was to define the clinical characteristics of metastatic NET undergoing surgical therapy with an apparent occult primary. Specifically, we aimed to determine the ability of surgical exploration to identify the primary site and characterize the clinicopathologic features of these tumors. Methods: Patients undergoing liver resection/ablation for NET metastases between 1/1988 and 8/2008 at a single institution were identified. Data on demographics, preoperative assessment, operative details, primary tumor status, and follow-up were collected. Results: In a cohort of 117 patients undergoing liver surgery for NET, 89 (76%) presented with liver metastases at the time of initial diagnosis. More than two-thirds were non-functional based on clinical or biochemical evaluation. Preoperative assessment included chest/abdominal/pelvis CT/MRI imaging in all patients and somatostatin receptor scintigraphy in one half. Of these, the primary tumor site was identified or suspected preoperatively in 65 (77%) whereas the primary remained unknown in 24 patients (27%). Of those with preoperative occult primaries, the tumor was detected intraoperatively in 67%. Histologic tumor type significantly impacted on pattern of primary tumor detection. Specifically, NET of pancreatic origin were signifanctly more likely to be detected preoperatively (p=0.008) compared to carcinoid tumors, whereas 10 of 11 (91%) tumors detected intraoperatively were of ileal carcinoid origin. When found, combined hepatic surgery and primary tumor resection was possible in the majority of cases.Conclusions: In patients undergoing surgical therapy for synchronous metastatic NET, the site of primary tumor is not known preoperatively in more than one forth of patients. However, surgical exploration will detect the majority of these, most of which will be of carcinoid origin.