The pattern of epithelial keratin expression in the normal anal canal has not been extensively defined and is a necessary prerequisite to the interpretation of alterations in these intermediate filaments in pathological anal epithelial lesions. Thirty‐five frozen tissue specimens of resected haemorrhoids were investigated immunohistolo‐gically for expression of 14 individual keratins (K) using a panel of 17 monoclonal antibodies.Perianal skinshowed basal expression of karatinocyte Ks 5, 14 and 17, and suprabasal expression of keratinocyte Ks 14, 10, 1 and 16.Anal squamous epitheliumshowed persistent basal K5 and 17, basal and suprabasal K4, 13 and 16 positivity, with sporadic expression of K1 and 10. The expression of simple epithelial keratins in squamous epithelium adjacent to the anal transitional zone varied with basal expression of K7, K8, K18 and K19 and sporadic suprabasal expression of K7 and K19. Theanal transitional zone (ATZ)expressed K19, as found in transitional epithelia elsewhere. The full thickness of epithelium was positive for the simple epithelial Ks 7, 8 18 and 19. Marked heterogeneity of keratinocyte keratin expression was seen. Basal layers expressed Ks 4, 13, 14 and 17 and variably K16, while suprabasal layers expressed Ks4 and 13, 14 and 17 and variably K16, while suprabasal layers expressed Ks4 and 13 and variably K14, 16 and 17. This anomalous expression of keratinocyte K4 and 13 has also been documented in transitional epithelium of the bladder. Theanal glands and ductsshowed a keratin distribution similar to the transition zone.Rectal columnar epitheliumexpressed simple keratins 7, 8 18 and 19. In addition, low levels of keratinocyte keratins were found as indicated by heterogeneous staining for K4, 13, 14 and 16. The overall pattern, particularly in the region of the anal transitional zone and immediately adjacent squamous and columnar epithelia, is of a flexible epithelial cell population able to express a range of keratins unrestricted by a particular morphological phenotype. In the light of these results, analysis of changes in keratin distribution within anal carcinomas may assist classification by providing information on the state of differentiation and histogenesis of these tumo