In the course of a population study in Italy, blood samples collected from 802 unrelated newborns and both their parents (when possible) have been examined for galactose-1-phosphate uridyltransferase (GALT) polymorphism. Electrophoresis and quantitative assay of GALT activity were not always sufficient for an accurate identification of the different GALT genotypes; segregation analysis provided better criteria for classification. A parent-child correlation coefficient for GALT activity equal to 0.107–0.155 was found when only the transmission of the normal allele was concerned, but the correlation rose to 0.618–0.682 when the Duarte and Los Angeles alleles were segregating. This confirmed the existence of a low (Duarte) and high (Los Angeles) activity variant. The overall validity of our genotype classification is supported by the good agreement between observed and expected mating types and segregations. The following gene frequencies were found for the different alleles: N = 0.9192, G = 0.0036, D = 0.0372 and LA = 0.0400.