Two 9-week-old, breast-fed premature infants developed acrodermatitis and hypozincemia because of low zinc content in their mothers' breast milk. All symptoms of zinc deficiency disappeared within seven days after the infants were treated orally with zinc and did not recur when zinc was discontinued after 11 months (infant 1) and three weeks (infant 3). After a subsequent term pregnancy, one of the mothers had lower breast milk zinc content (P < .025) and greater exponential decline (P < .025) of zinc content through 40 weeks of lactation compared with 34 control subjects. Her second infant (infant 2) had hypozincemia at 7 months of age but did not develop clinical zinc deficiency. When the mothers of the affected infants took oral zinc supplements, there was no increase in their breast milk zinc content. Zinc secretion into breast milk appears to be a controlled process that is independent of maternal zinc intake or serum zinc level. Breast milk may be low in zinc because of defective mammary secretion and this may lead to severe symptomatic zinc deficiency in premature infants.