A retrospective study of 53 patients with phenylketonuria (PKU), whose disease was managed with a low-phenylalanine diet, revealed a high incidence of iron depletion (as reflected by subnormal serum ferritin concentrations). Serum ferritin concentrations under 10 micrograms/l were found in one out of six infants aged 5-12 months. Concentrations under 16 micrograms/l were found in 16 of 22 children aged 1-3 years and in 11 of 25 children aged 4-12 years. Dietary iron, estimated from prescribed intakes of medical foods, exceeded the Canadian recommended nutrient intake, suggesting that low stores of iron may be secondary to reduced bioavailability and absorption of iron. These findings suggest that the current dietary management of PKU is associated with an increased risk for low iron stores. Investigators have reported an association in young children between iron-deficiency anaemia and both cognitive and motor disturbances. Children with PKU, already at risk of neurological damage because of phenylalanine neurotoxicity, may be at increased risk as a result of iron depletion. Serum ferritin as well as haemoglobin concentration should be monitored, along with plasma phenylalanine and tyrosine, to ensure optimum treatment of affected children.