Background: Neuroendocrine neoplasms (NENs) are a group of biologically and clinically heterogenous malignancies of different anatomic sites, the majority of which lie within the gastrointestinal (GI) tract. Prior literature has reported on the association between NENs and other primary malignancies (OPM), but these studies are limited by small sample sizes. We aim to further analyze the association of NENs and OPM on a larger scale using a population-based cancer registry. Methods: Primary malignancies with NEN features were identified from the Surveillance Epidemiology and Ends Results registry between 1975-2017. The histology/behavior of NEN included carcinoid tumor, neuroendocrine carcinoma (NEC), pancreatic endocrine tumor, atypical carcinoid tumor, and other (insulinoma, glucagonoma, gastrinoma, VIPoma, somatostatinoma and enterochromaffin cell carcinoid). First NEN observation from each patient was examined. Patients with NEN were grouped into 3 categories: NEN only, NEN first followed by OPM, and non-NEN first followed by NEN based on sequence number of primary cancer. Secondary malignancy sites were analyzed. Distribution of NEN between GI and non-GI sites was described. Demographics were compared by NEN sequence groups and between GI and non-GI sites. Results: 45,896 patients with NEN were analyzed (77.9% Caucasian, 47.0% male, median age 62.0 yrs). 65.7% of NENs were observed in GI sites. Within the GI tract, 31.3% were small intestine, 25.1% rectum, 16.6% pancreatobiliary, and < 11% in other GI locations. Age at NEN diagnosis was younger in those with GI NENs (median 60.0 vs 65.0, p < 0.001). 71.2% of patients had NEN only, 10.4% had NEN first followed by OPM, and 18.4% had a non-NEN primary followed by NEN. Mean age was 58.9 for NEN primary only, 61.0 for NEN primary first and 68.3 for non-NEN primary first (p < 0.0001). More Caucasian patients had non-NEN primary first (82.3%) compared to NEN primary only (76.9%) and NEN primary first (75.5%). There were no gender differences amongst the three groups. Carcinoid tumor histology was more prevalent in NEN primary first (78.6%) compared to NEN primary only (67.3%) and non-NEN primary first (66.6%), while NEC histology was more prevalent in NEN only (27.4%) and non-NEN first (29.3%) compared to NEN first. Among patients with NEN first followed by a second primary malignancy (SPM), 23.6% of females had a secondary breast malignancy and 28.9% of males had a secondary prostate malignancy. Conclusions: 28.8% of patients with NENs were found to have OPM, either preceding or following their NEN diagnosis. Of those with a SPM following NEN, breast and prostate were the most common sites for women and men, respectively. It is imperative that patients with NEN undergo age-appropriate cancer screening to help identify any concurrent malignancies. Further research is warranted to identify the timeframe in which they occur in relation to the NEN.