An infant formula based on a whey protein concentrate with a low phenylalanine content (3.5 %) has been tested as a protein source in the diet of an infant with hyperphenylalaninemia. Nitrogen balance studies performed at the age of 1.5, 7 and 12 months showed that feeding this formula instead of a conventionally humanized cow milk formula resulted in a positive nitrogen balance without a subsequent increase in the plasma phenylalanine level. Since the formula was well tolerated, has a high nutritive value and a better palatability than conventional phenylketonuria diets, as well as an amino acid composition similar to that of breast milk it seems to constitute an adequate diet for infants with hyperphenylalaninemia or transient tyrosinemia.