Amyloid deposits of the vitreous are usually associated with familial amyloidotic polyneuropathy (FAP). Various mutated forms of transthyretin (prealbumin) seem to form the main amyloid fibril component. Five Swedish patients, all with vitreous amyloidosis but no systemic symptoms or family history of amyloidosis, were examined using restriction fragment length polymorphism analysis. Genomic DNA was tested with a transthyretin complementary DNA probe. After cleavage with Nsi1, two restriction fragment length polymorphism markers of 5.1 and 1.5 kilobase were detected in the patients but not in the control subjects. These observations indicate the same methionine for valine substitution at position 30 of the transthyretin molecule in patients with vitreous amyloidosis as seen in Swedish patients with FAP as well as in patients with FAP from Japan and Portugal, and patients of Swedish descent with FAP from the United States.