The Finnish Bone Marrow Donor Registry was established 1992 to serve Finnish patients in urgent need of bone marrow transplantation. This study details the HLA antigen frequencies, including those of the A 19 subtypes, in the Finnish population. Large regional variations were found in antigen frequencies between the different geographical areas of Finland. In particular, antigens A9, B12, B35, Cw4 and DR3 display regional frequency deviations, but B7, B8 and B15 also exhibit regional variations. The present population is the largest (n = 10,000) ever HLA-typed in Finland. 97% of the donors were HLA-A-B-DR typed. Confirmation of the serological HLA type was performed by DNA typing on 3% of the donors in the registry. A potential donor was found for 52% of Finnish patients in need of a matched unrelated donor for a bone marrow transplantation. Due to the ethnic origin of the Finns, it is not easy to find suitable bone marrow donors for Finnish patients in worldwide registries. It is thus important to maintain a national bone marrow donor registry which recruits donors from all over the country.