As already described for cystic fibrosis and Friedreich ataxia, the incidence of PKU in Italy has been estimated by determining the increase of consanguineous marriages among 178 couples of PKU parents over the frequencies carefully established for the same marriages in the general Italian population for each of the 95 provinces during a 55-year period. The incidence estimated (between 1/15595 and 1/17815 according to two different formulas) is not very different from the incidence derived from screening programs (almost 1/12000). This indicates that the former method can be applied in Italy to the study of the incidence of other autosomal recessive disorders.