Pathologic and prognostic data of nine patients with mitochondrion-rich carcinomas (MRC) were compared retrospectively to data of 101 patients with conventional gastric adenocarcinomas. MRC was defined as a tumour composed predominantly, or entirely, of columnar adenocarcinoma cells with eosinophilic cytoplasm and a strong supranuclear immunoreactivity for antimitochondrial antibody. Electron microscopy confirmed supranuclear distribution of mitochondria in MRC while immunostaining pattern was irregular or absent in the remaining 101 cases. MRC exhibited a tubulopapillary or cribriform growth pattern with focal infiltration of neutrophils in the tumour stroma. Prominent necrosis was present including segmental and intraluminal "dirty necrosis", while mitotic and ki-67 proliferative rates were low. MRC showed immunohistochemical findings compatible with gastric differentiation (CK7+/CK20-/CDX-) When MRC were compared with non-MRC carcinomas, tumour size (< 4 cm vs >4 cm, P < 0.01) , frequency of lymph node metastases (11% vs. 80%, P < 0.01), low stage (I, II) at diagnosis (100% vs. 56%, P < 0.01), Goseki's group I (100% vs. 6%, P < 0.01), and better survival (0% vs. 70%, P < 0.01) differed significantly. Our results suggest that MRC of the stomach may be considered a low-grade malignancy with an excellent prognosis.