Collected experiences in the detection and treatment of phenylketonuria in a large children's hospital during the past 5 years have been described. Thirty cases of phenylketonuria have been confirmed. Early examination revealed phenylketonuria in three siblings of known cases. One case showing atypical features has been discussed in more detail. Four patients reported "positive to Phenistix" were found to be excreting phydroxy-phenylpyruvic acid. Two other false-positives were shown to be due to histidine derivatives. In an appendix, details are given of a simple routine method for the determination of plasma phenylalanine.