Heterotopic ossification of gastric adenocarcinomas is very rarely observed in humans. In rats such lesions could be induced by ingestion of nitroso compounds. We describe the case of a 70-year-old patient with an extended, initially spleen-infiltrating tubular adenocarcinoma of the stomach with a mucinous component, heterotopic ossifications and pronounced regressive alterations after neoadjuvant chemotherapy. Various speculations regarding the pathogenesis of heterotopic ossifications in gastrointestinal adenocarcinomas are discussed.