OBJECTIVE To evaluate the long-term efficacy and safety of diflunisal in late-onset familial amyloid polyneuropathy (FAP) in a Japanese endemic area. METHODS Consecutive six FAP patients (mean age: 65.8 ± 7.3 years) with a transthyretin (TTR) Val30Met mutation from an endemic area of late-onset FAP were prospectively recruited to an open label study with oral diflunisal (250 mg twice a day). We evaluated clinical symptoms, Kumamoto FAP score, modified body mass index (mBMI), Medical Research Council sum score, nerve conduction studies (NCS), electrocardiogram (ECG), ECG Holter monitor test, echocardiography, and (123)iodine-metaiodobenzylguanidine ((123)I-MIBG) myocardial scintigraphy. RESULTS One patient ceased to take diflunisal because of hematuria which was reversible. The other five patients were treated with diflunisal for 3-5 (4.4 ± 0.9 years) years. Autonomic symptoms (orthostatic hypotension and gastrointestinal symptoms) disappeared after treatment in two of the four patients with the symptoms. Delayed heart to mediastinum ratio on (123)I-MIBG imaging, a marker of cardiac postganglionic sympathetic nerve function, increased during the three-year treatment. mBMI was maintained through observation period. While, motor and sensory symptoms, Kumamoto FAP scores, and data on NCS gradually deteriorated. CONCLUSION Diflunisal might be effective especially for autonomic dysfunction in late-onset FAP with a TTR Val30Met mutation.