Data are now available on 9 pedigrees in detail and 4 pedigrees as lod scores only. Linkage to HLA is significant (Z = 5.53 at recombination rates of 0.223 in males and 0.327 in females). Tight linkage is excluded. Nine pedigrees which appear to be typical olivopontocerebellar atrophy (OPCA I) have recombination rates of 0.150 in males and 0.300 in females. The remaining 4 pedigrees are clinically atypical or include discrepant data and give no evidence for linkage. The symbol SCA1 is proposed for a locus on chromosome 6 (loosely linked to HLA), at which at least one allele produces OPCA I (Menzel type). It is not yet clear whether other clinical types are determined by alleles at different loci, although this is suggested by several pedigrees, including a Danish pedigree of OPCA with dementia. Linkage evidence will be decisive in delineating the ataxias.