A case of familial amyloidotic polyneuropathy (FAP) is reported in which peripheral nerve and skeletal muscle biopsies were obtained from the right leg to assess the severity of the relatively late-onset but rapid-evolving neuropathy. The present paper deals with some remarkable features found in the muscular biopsy, taken from peroneus brevis and extensor digitorum longus muscles. Several fibers contained amyloid masses characterized by Congo red positivity and birefringence on polarized light microscopy: histoenzymologic staining revealed that these fibers were always type 2B and appeared grossly hypertrophied. The presence of amyloid inside the muscle fibers was possibly dependent on the internalization of capillaries leading to direct deposition of amyloid fibrils into the sarcoplasm. Fiber vascularization occurred independently of fiber splitting, which appeared to be frequent in both muscles and was characterized by unusual segmental changes in the histochemical properties of the daughter fibers with respect to those of the parent fiber. Target/targetoid and degenerating areas were also observed in a large number of type 2 fibers, usually in close relationship with segmental splitting phenomena. These findings were interpreted as possible secondary myopathic changes accompanying chronic denervation-reinnervation episodes in the course of FAP.