SummaryA comparison has been made of the birthplaces of the parents and grandparents of a series of patients with phenylketonuria with those of the parents and grandparents of three control series of patients with Hirschsprung's disease, congenital dislocation of the hip and coeliac disease. The index patients in each series have been restricted to those referred to The Hospital for Sick Children from homes in south‐east Englànd.It was found that a high proportion of the parents and grandparents of the phenylketonuric patients were born in Ireland or west Scotland. The frequencies, relative to the control patients, of phenylketonurias with grandparents born in Ireland or west Scotland, suggest that the frequency of the gene for phenylketonuria is about four times as high in the population of these parts of the British Isles as in the population of south‐east England.It is also noteworthy that several phenylketonuria patients had Lithuanian or Polish ancestry, and one patient was of gypsy descent on both sides of the family.We wish to thank Prof. A. A. Moncrieff, Dr J. A. Fraser Roberts and Dr A. C. Stevenson for their encouragement during the investigation and their criticism of this paper, also Dr M. Fraccaro and Dr J. Stern for their help with the references, and Prof. H. Utena for his information about Japan. One of us is indebted to the Research Committee of The Hospital for Sick Children for financial support in the early part of this investiga