Eighty-three infants with classical phenylketonuria have been born in Norway since 1979. The treatment of these children is centralized at the National Hospital in Oslo. Seventy-four have been breastfed in combination with a phenylalanine-free protein substitute. Dietary treatment was commenced in hospital between 5 and 33 d of age (mean 14 d). Normalization of serum phenylalanine (below 400 pmol/l) took between 1 and 35 d (mean 8 d). The period of breastfeeding lasted from 1 to 16 mo (mean 7 mo). Growth (weight, length and head circumference) fell within the normal range for age on the Norwegian growth chart.