The highest incidence rates of phenylketonuria (PKU) have been observed in Ireland and Scotland. Parents heterozygous for PKU in Norway differ significantly from the general population in the Rhesus, Kell and PGM systems. The parents investigated showed an excess of Rh negative, Kell + and PGM type 1 individuals, which makes them similar to the present populations in Ireland and Scotland. It is postulated that the heterozygotes for PKU in Norway are descended from a completely assimilated sub‐population of Celtic origin, who came or were brought here, 1000 years ago. Bronze objects of Western European (Scottish, Irish) origin, found in Viking graves widely distributed in Norway, have been taken as evidence of Vikings returning with loot (including a number of Celts) from Western Viking settlements. The continuity of residence since the Viking age in most habitable parts of Norway, and what seems to be a nearly complete regional relationship between the sites where Viking graves contain Western imported objects and the birthplaces of grandparents of PKUs identified in Norway, lend further support to the hypothesis that the heterozygotes for PKU in Norway are descended from a completely assimilated subpopulation.The remarkable resemblance between Iceland and Ireland, in respect of several genetic markers (including the Rhesus, PGM and Kell systems), is considered to be an expression of a similar proportion of people of Celtic origin in each of the two countries. Their identical, high incidence rates of PKU are regarded as further evidence of this. The significant decline in the incidence of PKU when one passes from Ireland, Scotland and Iceland, to Denmark and on to Norway and Sweden, is therefore explained as being related to a reduction in the proportion of inhabitants of Celtic extraction in the respective population