An investigation of the fat content of breast milk was carried out in 71 Brazilian women from a poor socio-economic background and in 10 from a high social class. All were breastfeeding their infants without supplements. Of the 81 women, 28 "poor" women were assessed as "malnourished" according to weight-for-height criteria, and varying proportions as "malnourished" when other criteria were used. The milk of "malnourished" mothers, even when malnutrition was severe, was slightly (but not statistically significantly) richer in fat than that of well-nourished women. The proportion of primiparous women was similar in both nutritional groups, and multiparity up to five pregnancies was associated with an increased fat concentration. Infants from all groups gained weight in a similar "normal" way, demonstrating that poor maternal nutritional status in this population had no adverse effect on the adequacy of lactation. It was concluded that the promotion of exclusive breastfeeding for at least 4 months was a realistic objective of the Brazilian National Breastfeeding Programme.