The incidence and prevalence of upper gastrointestinal neuroendocrine tumors (NETs), including gastric NETs (GNETs) and duodenal NETs (DNETs), have been gradually increasing. These trends may be associated with the increased use of health checkups, which includes upper endoscopy, in conjunction with better disease recognition. However, the clinical factors associated with GNETs and DNETs remain unknown; previous studies revealed discrepancies. Recently, metabolic disorders have been indicated as potential factors that are associated with GNETs and DNETs. This review summarizes the results of previous studies and briefly introduces the results of a recent Korean multicenter study on the factors associated with GNETs and DNETs.