In a Portuguese-American family with hereditary amyloid neuropathy (familial amyloidotic polyneuropathy), onset was in the seventh decade in all affected relatives. Another unusual characteristic was their origin from the Portuguese island of Madeira. In spite of this, the mutant transthyretin (TTRMet30) (the same variant prealbumin that is the circulating precursor of AFP protein in the classic Portuguese patients) could be found in the propositus' plasma. In addition, three other asymptomatic relatives (ages 90, 73, and 48) were shown to carry the mutation. Late onset and incomplete penetrance, at a clinical level, raise problems for presymptomatic detection of mutant TTR, as these tend to cluster in families. When counseling asymptomatic heterozygotes, we must consider intra-familial correlation in age-of-onset, and the distribution of age-of-onset including age of unaffected heterozygotes. This family poses interesting questions regarding pathogenesis of this degenerative process and the influence of other genetic factors, such as modifiers, epistasis, and polymorphism of the TTR genes or their regulators. A cis-effect of a gene linked to the mutant gene, decreasing the synthesis of the mutant TTR and keeping a sufficient amount of the normal one in circulation, or producing some cofactor for TTR, could also explain late onset and apparently incomplete penetrance; the occasional finding of classic forms in these families would be the result of recombinatory events.