The Hp groups of 15,601 unrelated, adult Swedish males and females have been determined. The results have been recorded according to the county (län) of residence of the individuals. The average Hp1 gene frequency of the total population was found to be 0.382. Some heterogeneity between the counties was apparent, but it was possible to delimit a few larger, more homogeneous regions. Thus, the northern half of Sweden showed uniformly low Hp1 gene frequencies with a mean of 0.356. Most of the counties of the western part of southern Sweden constituted a fairly homogeneous region with a mean Hp1 frequency of 0.387. The central areas of Sweden and the southeastern part which, according to previous blood group investigations, make out a relatively homogenous region, displayed more marked variations with respect to the Hp groups: the Hp1 gene frequency was generally found to be low, about 0.35, in the southeastern corner, but higher and more variable towards the center with values above 0.40 in some counties. Uncommon variants were observed in 0.07% of the total material and Hp 0 in 0.22%.