SYNOPSISThe pathology of 21 cases of muco-epidermoid carcinoma of the anal canal is described. These tumours are defined as squamous or transitional cell carcinomas containing areas which secrete mucin of undoubted epithelial origin within the clumps of tumour cells. They are found in that part of the anal canal just above the dentate line where the epithelium is variable in type and represents an embryological transition between rectal and squamous mucous membrane. The histogenesis of these tumours is discussed and the opinion given that they arise from the surface epithelium of the upper anal canal rather than from the anal glands.Since Stewart, Foote, and Becker (1945) described muco-epidermoid tumours of the salivary glands growths with a similar histological structure have been described in the oral cavity (Smith, Broadbent, and Zavaleta, 1954), uterine cervix (Hellweg, 1957), urinary bladder (Feyrter, 1956), oesophagus (Azzopardi and Menzies, 1962), and anal canal (Kay, 1954 In all but three of these cases the rectum, anal canal, and peri-anal skin were removed by synchronous combined excision. The surgical specimens were prepared in the fresh state by cutting along the anterior aspect and stretching them out on a metal frame. The specimens were photographed and an accurate assessment made of the exact site of the primary tumour as well as the extent of local and lymphatic spread. In particular the relationship of the tumour to the dentate line or line of the anal valves was assessed as accurately as possible.Of the three tumours not treated by excision of the