A 44-week clinical study was carried out on a large sample of the Italian infant population to measure the incidence of diaper (napkin) dermatitis and to identify the factors related to the development of diaper dermatitis. Factors such as age, sex, the presence of atopic dermatitis, the general state of health of the child, the use of drugs and the type of diaper used were recorded. At the end of the test, 2169 clinical dermatological examinations were performed. The frequency of episodes of diaper dermatitis was 15.2%. Statistical correlations between diaper dermatitis and age, presence of atopic dermatitis, and health conditions were found.