The ability of infants with protein-energy malnutrition to absorb iron was assessed using the serum iron response to a dose of ferrous sulfate providing 3 mg elemental iron per kg body weight. Responses were grouped as flat (delta serum Fe less than 30 microgram/dl), intermediate (30 to 100 microgram/dl), and normal (greater the 100 microgram/dl). Of 25 consecutively admitted children studied, seven had a flat, five an intermediate, and 13 a normal curve (mean delta serum Fe: 10 microgram/dl, 66 microgram/dl, and 175 microgram/dl, respectively). There were no differences among the three groups in hematocrit, fasting serum iron or transferrin saturation, severity of malnutrition, or evidence of other malabsorption sufficient to explain these differences. Although hematocrits, fasting serum iron, and transferrin saturations did not change appreciably during nutritional rehabilitation, all children with initially abnormal responses subsequently had normal tests.