BACKGROUND Hereditary amyloidogenic transthyretin (ATTR) amyloidosis is an autosomal dominant, multi-systemic and progressive disorder characterised by polyneuropathy, cardiomyopathy and dysautonomia to varying degrees. In Ireland, the p.Thr80Ala mutation has been well documented, but little has been reported about a second variant, the p.His110Asp mutation first discovered in a family native to county Cork. Here we elaborate on the phenotype of this recently identified mutation using an extended pedigree of the original kindred and include for the first time a second affected family. METHODS Patients attending our centre with confirmed or suspected ATTR amyloidosis as a result of a p.His110Asp mutation were identified. Detailed chart reviews and patient interviews were completed. Data on symptoms, examination findings, neurophysiology, histology, biochemistry, and cardiac investigations were gathered. A large extended pedigree was plotted. RESULTS A total of 17 members across four generations of one kindred, and 2 members of a previously unreported family were identified. A phenotype of progressive late-onset polyneuropathy with cardiac involvement was common to both families. An early manifestation was carpal tunnel syndrome, preceededing neuropathy by many years. Gastrointestinal and urinary symptoms were common. CONCLUSIONS We describe a wider phenotype of the p.His110Asp mutation of transthyretin in two Irish families. Importantly, we describe cardiac involvement which was not previously emphasised. The discovery of a new unrelated family highlights the importance of clinical suspicion even in those without known family history. We suggest that this second important transthyretin mutation should be considered in patients of Irish origin.