In a prospective study of thirty-seven infants presenting with dermatitis in infancy it was found that pruritus, a positive family history of atopy, and the age of onset of skin lesions were of little value in distinguishing atopic dermatitis from infantile seborrhoeic dermatitis. The most useful distinguishing features were the increased incidence of lesions on the forearms and shins in atopic dermatitis, and in the axillae in infantile seborrhoeic dermatitis. The onset of skin lesions solely in the napkin area favoured a diagnosis of infantile seborrhoeic dermatitis, but when more than one site was involved the distinction was less clear. Infantile seborrhoeic dermatitis carried a much better prognosis than atopic dermatitis. In thirty-six out of thirty-seven infants the definitive diagnosis could be made with confidence on clinical grounds by the age of I year, but was often difficult at first presentation in early infancy.