To determine whether failure to thrive in a breast-fed infant could be attributed to altered milk production or composition from a mother who consumed a self-imposed energy- and protein-restricted diet.We evaluated the changes in growth and body composition, dietary intakes, and milk production and composition in a mother-infant pair throughout the first postpartum year.The Children's Nutrition Research Center Metabolic Research Unit.A breast-feeding mother-infant pair.Body composition was measured by total body electrical conductance, dietary intakes by food records, milk production by the test weighing procedure, and milk composition by proximate analyses.A marked decline in the infant's linear and ponderal growth rates occurred when the mother consumed an energy- (20 kcal.kg-1.d-1) and protein- (0.7 g.kg-1.d-1) restricted diet. The retardation in body weight gain was associated with an arrest of body fat, but not lean body mass, accretion. Maternal milk production showed positive relationships with maternal dietary energy (p<0.01, r = 0.93) and protein (p<0.05, r = 0.83) intakes. Milk composition reflected changes consistent with those of weaning rather than a sequela of the mother's diet.We concluded that failure to thrive in a breast-fed infant could be attributed to reduced milk production in conjunction with maternal dietary energy- and protein-restriction and that an assessment of maternal dietary intakes is essential in an evaluation of the breast-fed infant with failure to thrive.