Six breast-fed infants living in a dry desert climate area presented at ages 4-11 days with severe dehydration. In all cases, dehydration was associated with inadequate breast milk production by the mothers. In contrast to earlier reports on dehydration in breast-fed infants, five of the mothers were from a low socioeconomic background and three were multiparas, including two mothers who had previous experience with breast-feeding. In two infants severe bacterial infections were documented. Two mothers had small retracted nipples. As in earlier reported instances, sodium concentration in breast milk was elevated in all five mothers in whom it was measured, and three babies had severe hypernatremia. Successful relactation was achieved in three cases with a decrease in the milk sodium concentration. These data demonstrate that severe dehydration in breast-fed infants may occur as early as the first week of life and may affect even infants of experienced multiparous mothers who are well motivated for breast-feeding. It seems that hypernatremia in these infants is secondary to poor fluid intake and increased insensible water loss rather than to elevated milk sodium, since there was no direct correlation between milk sodium concentration and serum sodium levels. Following fluid resuscitation, relactation can be achieved in motivated mothers.