AbstractGastric carcinoma is one of the most common types of cancer in Taiwan. Somatic mitochondrial DNA (mtDNA) alteration in gastric carcinoma and its association with clinicopathologic features remain unclear. When we used polymerase chain reaction (PCR) and direct sequencing, 15 of the 31 (48%) gastric carcinomas displayed somatic mutations in the D‐loop region, a hot spot for mutations in mtDNA of human cancers. Ten (67%) cancers with the somatic mutations in the D‐loop had insertion or deletion mutations in nucleotide position (np) 303–309 in the mononucleotide repeat region. One carcinoma carried tandem duplication and triplication flanked by mononucleotide repeats starting at np 311 and 568, respectively, in the D‐loop. We also detected the common 4,977‐bp deletion in 17 (55%) of the noncancerous tissue samples, but only in three (9%) carcinomas. Moreover, we quantified the mtDNA content using a competitive PCR technique and found that mtDNA depletion occurred in 17 (55%) of the gastric carcinomas. Although no significant association was found between clinicopathologic features and the mtDNA mutations in the D‐loop, mtDNA depletion was observed significantly in the ulcerated, infiltrating (Borrmann's type III) and diffusely thick (Borrmann's type IV) types of gastric carcinomas (P = 0.018). Our results suggest that somatic mtDNA mutations and mtDNA depletion occur in gastric cancer and that mtDNA depletion is involved in carcinogenesis and/or cancer progression of gastric carcinoma. © 2005 Wiley‐Liss, Inc.