The transthyretin (TTR) amyloid diseases, representative of numerous misfolding disorders, are of considerable interest because there are mutations that cause or suppress disease. The Val³⁰ → Met³⁰ (V30M) TTR mutation is the most prevalent cause of familial amyloid polyneuropathy in heterozygotes, whereas a Thr¹¹⁹ → Met¹¹⁹(T119M) mutation on the second TTR allele protects V30M carriers from disease. Here, we show that the incorporation of one or more T119M TTR subunits into a predominantly V30M tetramer strongly stabilized the mixed tetramer against dissociation. Dissociation is required for amyloid formation, so these findings provide a molecular explanation for intragenictrans-suppression of amyloidosis. The data also suggest a potential therapeutic strategy, provide insight into tissue-specific deposition and amyloid composition, and support the validity of the amyloid hypothesis in human disease.