Serial small intestinal biopsies related to withdrawal and challenge with cows' milk are described in 5 infants with cows' milk protein intolerenace. On the basis of these serial observations the existence of a cows' milk-sensitive enteropathy in infancy is clearly established. The cows' milk-sensitive mucosal lesion varied in its severity ranging from a partly flat mucosa to a mild degree of partial villous atrophy. Intraepithelial lymphocyte counts rose after a positive milk challenge, but on only one occasion to a level outside the normal range. The technique of serial biopsies related to dietary milk enables a firm diagnosis of cows' milk intolerance to be made upon the basis of a single milk challenge. Clinically this appears to be a temporary condition which disappears by the age of 2 years.