We earlier reported that heat shock protein 105 (hsp105), which we identified by serological analyses of recombinant cDNA expression libraries (SEREX), was overexpressed in human colon and pancreatic adenocarcinoma. We went on to examine hsp105 expression in various colorectal cancers and adenomas, using immunohistochemical analysis. The 44 of 53 patients with colorectal cancers (83.0%) and only 2 of 21 (9.5%) with colorectal adenomas had an evident overexpression of hsp105, which means that overexpression of hsp105 is a late event in the colorectal adenoma-carcinoma sequence. Subsequently, we asked if hsp105 was overexpressed in other human tumors. During immunohistochemical studies, we discovered that overexpression of hsp105 occurred not only in cases of colorectal and pancreatic adenocarcinoma, but also thyroid, esophageal, breast, and bladder carcinoma and islet cell tumor, gastric malignant lymphoma, pheochromocytoma, and seminoma. On the other hand, hsp105 was evidently overexpressed only in the testis in human adult normal tissues. Thus, hsp105 is a useful marker of a variety of human tumors and hsp105 may prove to be a target molecule for designing anti-tumor immunotherapy.