Zinc deficiency was diagnosed in a breast-fed mature infant and her sister. In both infants the characteristic dermatitis appeared on the face and buttocks around 10 weeks of age. It responded rapidly to zinc supplements. Their mother´s serum zinc level was slightly low but her milk was found to be remarkably low in zinc. Oral zinc supplementation could correct only her serum zinc level but not her low breast milk zinc level. Therefore the mother´s deficiency in the transfer process of zinc from serum to breast milk was suspected as a cause of the skin changes in her children. These cases indicate that even mature infants, who feed exclusively on mother´s milk, run a risk to develop zinc deficiency, if the concentration of zinc in the breast milk is very low.