PURPOSE: We investigated the clinicopathologic information of patients with gastric cancer with multiple primary cancers (GC-MPC) of three or more sites. MATERIALS AND METHODS: Between 1995 and 2009, 105,908 patients were diagnosed with malignancy at Severance Hospital, Yonsei University Health System. Of these, 113 (0.1%) patients with MPC of three or more sites were registered, and 41 (36.3%) of these were GC-MPC. We retrospectively reviewed the clinical data and overall survival using the medical records of these 41 GC-MPC patients. We defined synchronous cancers as those occurring within 6 months of the first primary cancer, while metachronous cancers were defined as those occurring more than 6 months later. RESULTS: Patients with metachronous GC-MPC were more likely to be female (p=0.003) and young than patients with synchronous GC-MPC (p=0.013). The most common cancer sites for metachronous GC-MPC patients were the colorectum, thyroid, lung, kidney and breast, while those for synchronous GC-MPC were the head and neck, esophagus, lung, and kidney. Metachronous GC-MPC demonstrated significantly better overall survival than synchronous GC-MPC, with median overall survival durations of 4.7 and 14.8 years, respectively, and 10-year overall survival rates of 48.2% and 80.7%, respectively (p<0.001). CONCLUSION: Multiplicity of primary malignancies itself does not seem to indicate a poor prognosis. The early detection of additional primary malignancies will enable proper management with curative intent.