The Cohen syndrome is a rare autosomal recessively inherited disorder. Contrary to many case reports published elsewhere, the phenotype is uniform in Finland including nonprogressive mental and motor retardation, typical dysmorphic features, granulocytopenia and marked ophthalmological changes. By linkage analysis in five Finnish multiplex nuclear families, the COH1 locus for the Cohen syndrome was recently assigned to a 10-cM region between loci D8S270 and D8S521 on the long arm of chromosome 8. Here we present results of linkage disequilibrium and haplotype analysis in an extended panel of 16 Finnish COH1 families using new markers localized in the COH1 region. By inferring historical recombinations in conserved haplotypes the COH1 gene was assigned in the region of marker loci D8S1808, D8S1762 and D8S546. Calculations of genetic distances based on linkage disequilibrium suggest that the most likely localization of COH1 is in the immediate vicinity of marker locus D8S1762. Haplotype analysis suggests the occurrence of one main COH1 mutation and possibly one or two rare ones in Finland. This information will be useful in the positional cloning of the gene.