The aim of this study was to determine the dimensions of corneocytes collected from healthy dogs and cats, and from dogs suffering from atopic dermatitis. Samples were collected from the inner pinna, lateral thorax and the groin. D-Squame adhesive discs were used to collect corneocytes from the skin surface and image analysis software was used for measurements. Two differently shaped cells were identified in both animal species. The most common cell type was polygonal, often hexagonal or pentagonal and regular while the second type was smaller, elongated and variable in size and shape. The polygonal cells are corneocytes which probably originate from the interfollicular epidermis. The mean diameter and surface area for healthy canine polygonal corneocytes were 38-43.5 microm and 1092-1436 microm(2). The equivalent Figures for cats were 39.6-48.5 microm and 1183-1772 microm(2). Feline polygonal corneocytes were generally larger than those of the dog. Both feline and canine polygonal corneocytes collected from the ear were generally smaller than those from other body sites. Atopic canine polygonal corneocytes collected from the groin were significantly smaller than healthy groin corneocytes. In healthy dogs the mean length, breadth and surface area of elongated cells were 26.6-35.9 microm, 7.6-10.3 microm and 168.6-240.2 microm(2). The equivalent values for cats were 20.0-37.8 microm, 6.8-9.9 microm and 117.6-245.6 microm(2). The exact nature of the elongated cells is not known but they may be cell fragments or folded corneocytes. They were more common in densely haired skin suggesting the hair follicle as their origin.