Longitudinal splitting of muscle fibres has been studied in the biopsies of eighteen patients with neurogenic disorders, and of twenty with polymyositis. In neurogenic disorders splitting predominantly affects hypertrophied fibres, and is probably due to mechanical overload induced by normal loads imposed on a weakened muscle. A similar phenomenon occurs in hypertrophied fibres in chronic polymyositis. However, in acute, or active polymyositis an appearance resembling fibre splitting can result from sequestration of necrotic segments within a fibre and also from regeneration occurring within intact sarcolemmal tubes after segmental sub-endomysial necrosis. These different processes, which can be distinguished by light and ultrastructural criteria, are important compensatory factors in neuromuscular disorders.