A significant minority of degenerative dementias lack distinctive inclusion bodies, plaques or tangles on pathological examination. Half of these cases have a positive family history of dementia. We have studied the largest published family with such a dementia and mapped the disease locus to a 12 cM region of chromosome 3 spanning the centromere. Haplotype analysis demonstrates a common region shared between all affected individuals between the markers D3S1284 and D3S1603. Like a number of other late onset neurodegenerative diseases, the disease presents at an earlier age when paternally inherited.