Keratin 20 is a recently identified keratin protein distributed particularly in the epithelial cells of the gastrointestinal tract. In this study, keratin 20 was immunohistochemically evaluated in 788 epithelial tumors of different organs. Keratin 20 was consistently present in colonic adenocarcinomas and their metastases in lymph nodes, liver, lung, and ovaries; most primary and metastatic colon carcinomas showed high numbers of positive cells independent of their level of differentiation. Adenocarcinomas of the upper gastrointestinal tract, pancreas, and cholangiocarcinomas showed variable reactivity. Hepatocellular carcinomas and carcinoid tumors often showed focal reactivity limited to scattered tumor cells. In contrast, keratin 20 was virtually absent in primary adenocarcinomas of lung, ovaries, and endometrium. Notable exceptions among ovarian tumors were the mucinous neoplasms that showed variable, sometimes significant keratin 20 reactivity. Transitional cell carcinomas irrespective of grade were usually positive, whereas most prostatic and renal adenocarcinomas were negative or showed only single positive cells. Typically negative were squamous cell carcinomas of all organs and carcinomas of the breast. Merkel cell carcinomas of the skin showed consistent reactivity, whereas small cell carcinomas of the lung were negative. On the basis of these observations, keratin 20 seems to be a suitable adjunct marker to evaluate the primary origin of carcinomas in specific contexts, especially to separate adenocarcinomas of gastrointestinal versus nongastrointestinal origin.