Colonic mucosa typically expresses cytokeratin (CK) 20 but not CK7. This CK20+/CK7- profile has been used to distinguish colonic adenocarcinoma from others arising in the lung, breast, or genitourinary tract. CK7 expression in colorectal adenocarcinoma has been reported to be rare, and when present, a metastatic origin needs to be excluded. However, we have observed a higher frequency of CK7 positivity in rectal adenocarcinomas. Paraffin sections of 42 rectal tumors (7 adenomas and 35 adenocarcinomas), 11 colonic adenocarcinomas proximal to the rectum, and 18 nonneoplastic rectoanal mucosa were randomly selected and immunostained for CK7 and CK20 with a standard avidin-biotin complex method. Immunoreactivity was recorded semiquantitatively. Cytoplasmic CK7 immunoreactivity was noted in 29 of 42 (69%) rectal glandular neoplasms (3 of 7 adenomas [43%] and 26 of 35 adenocarcinomas [74%]) and 9 of 18 (50%) nonneoplastic anorectal mucosal samples. In contrast, only 3 of the 11 (27%) colonic adenocarcinomas proximal to rectum were CK7 positive. Because of the relatively higher frequency of CK7 expression in rectal epithelial neoplasms, when CK7 is applied to distinguish primary colorectal versus metastatic origin, its reactivity should be interpreted with caution and should not be used as the sole evidence for excluding a rectal primary, particularly in tumors involving the rectal or pelvic region.