Background: Although cases of multiple primary malignant neoplasms are increasing, reports of more than three or four primary metachronous malignant neoplasms are extremely rare. Moreover, very few publications have provided a genetic mutational analysis or have evaluated risk factors associated with such neoplasms. We present an extremely rare case of nine primary malignant lesions in a man who was successfully treated. We also report on microsatellite stability status, analyze risk factors, and discuss the relevant literature. Case presentation: Between 67 and 73 years of age, a male patient developed nine primary metachronous malignant lesions: Three were located in the esophagus, two in the stomach, two in the colorectum, one in the prostate gland, and one in the external ear canal. The patient's clinical history included hypertension, atrial fibrillation, an acoustic schwannoma, and heavy smoking. The lesions were diagnosed during regular screening over a six-year period. He was successfully treated with surgery (both open surgical and endoscopic resection of lesions) and adjuvant chemotherapy. Immunohistochemistry and mutational analysis showed that the lesions were microsatellite stable, and the KRAS, BRAF, p53, and nuclear β-catenin status was not uniform among the lesions. Conclusions: Given that the presence of more than three or four neoplasms is extremely rare, the present case of nine primary malignancies with no associated microsatellite instability and no apparent predisposing hereditary conditions, is extraordinary. Our case study shows that it is possible for up to nine sporadic neoplasms to occur, and efficient disease management requires diligent screening and early detection.