Familial amyloidosis, Finnish (FAF), is an autosomal dominant form of systemic amyloidosis with lattice corneal dystrophy and progressive cranial neuropathy as principal clinical manifestations. We have shown that the novel amyloid fibril protein found in these patients is an internal degradation fragment of gelsolin, an actin-binding protein, and that it contains an amino acid substitution, asparagine for aspartic acid at position 15, that is due to a guanine-to-adenine transversion corresponding to codon 187 of human plasma gelsolin cDNA. To test that this mutation cosegregates with the disease high-molecular-weight genomic DNA was isolated from autopsied tissues or lymphocytes of 23 patients, 6 healthy relatives and 20 unrelated healthy control persons. Specific fragments were amplified with the polymerase chain reaction for oligonucleotide hybridization analysis using the slot-blot technique. The guanine-to-adenine transversion was found in all FAF patients tested, but in none of the control subjects. Our results show that the mutation (G to A) cosegregates with the disease phenotype, and that the slot-blot analysis can be used as a diagnostic assay, including prenatal evaluation.