Histocompatibility antigens were defined in 25 members of a Finnish family in which Creutzfeldt-Jakob disease (CJD) was diagnosed clinically in 4 cases and neuropathologically in 3 cases; 3 further cases had a history of presenile dementia. 2 HLA haplotypes were defined in 3 patients and deduced in a further 3 cases; only one haplotype could be deduced in 2 patients. CJD was not linked with a single haplotype, but at least 7 out of 8 patients with this disease apparently shared the HLA antigens A28 and B8.