In the county of Västerbotten in northern Sweden, a large number of individuals with the rare blood group p have been found. The ancestors of all known 31 cases were studied genealogically, and the data showed that about one half of all cases (15 out of 31 in 9 out of 20 families) could be traced to one gene source in the north-eastern part of the county in the middle of the 17th century. In two of the families the parents were first cousins and in three they were second cousins. In the 18th and 17th centuries genealogical connections between the parents were found in another 10 of the families