An eruptive condition seen on certain mucous membranes of dogs occurs frequently but often asymptomatically. Lesions have been found to be present in over 60 percent of male dogs. The infection is probably due to a canine herpes virus and has been termed conveniently but inaccurately "dog pox". In many cases lesions are present without observable clinical signs but severe irritation, especially related to lesions in the lower bowel, can give rise to marked symptoms which may be confused with abnormality of the para-anal sacs, or to chronic diarrhoea of a type suggestive of colitis. The incidence, sites of infection and possible syndromes which may be ascribed to the condition are briefly discussed on a clinical basis only.