We report a family in which the proband died of clinically typical, neuropathologically verified Creutzfeldt-Jakob disease; her still-living mother suffers from a progressive dementia of many years' duration, and her maternal grandfather died after a similar illness. The proband, her mother, and two of three young first-degree relatives all have an identical insert mutation in the PRNP gene consisting of a twice-repeated 24-nucleotide sequence in the region between codons 51 and 91.