A fractal dimension of skin structure was determined in order to evaluate the pathological damages evoked by percutaneous absorption promoters such as d-limonene and Azone in the hairless rat. As a model dermatological system, indomethacin hydrogels containing each of these enhancers were applied to the hairless rat abdominal skin. The application site of the skin was excised, fixed with glutaraldehyde and osmic acid, and then lyophilized. Boundary lines between the outermost lipid layer and the underlying protein parts in the skin cross section were taken by the CCD camera as image data, and these images were fed into a computer to estimate the fractal dimension. The boundary line of the skin cross section was observed to be a typical fractal morphology and the structure changes of skin evoked by the promoters could be quantified as a non-integral fractal dimension. A linear relationship between the fractal dimension and pathological irritation score was clearly observed, suggesting the usefulness and reliability of the fractal dimension as a novel method for evaluating the skin damage.