Abstract: Diaper dermatitis may result after repeated or prolonged contact of skin with urine and feces. A hairless mouse model was used to elucidate the role of urine in this process. The results of this work suggest that an important function of urine in the etiology of diaper dermatitis is to increase the pH of the diaper environment by breaking down urea in the presence of fecal urease. This rise in pH increases the activities of fecal proteases and lipases, which can damage skin. Urine can also increase the permeability of diapered skin to irritants and can directly irritate skin when exposure is prolonged.