Comparative bioptic and electrophysiological investigations were performed in 30 cases of polyneuropathy of varying aetiology. Axonal and demyelinating neuropathies were distinguishable concordantly on the basis of both investigations. Maximum motor nerve conduction velocity (NCV) and compound potential (CP) were regularly examined in several nerves of each patient. Low (NCV) values (critical limit 25 m/sec) and long-standing high CP values were characteristic of demyelinating processes, whereas axonal neuropathies showed a slight to moderate decrease in NCV with early decrease in CP, often rapidly followed by loss of nerve excitability. Nonclassifiable stages and diseases are discussed. In biopsies, ultrastructural examination in particular, not only allowed the differentiation of primary axonal and primary demyelinating lesions and the respective neuropathic syndromes, but permitted moreover, further classification of these heterogenous groups of neuropathies. According to the sequence of lesions of the nerve fibre classes on the one hand, and to the nature of the primary lesion of the nerve fibres on the other hand, the following types of neuropathies were distinguished: in axonal neuropathies of the myelinated fibres, dystrophic and disintegrative forms were differentiated; axonal neuropathies of unmyelinated fibres correlated with sensory neuropathies. Demyelinating neuropathies were represented by hypertrophic, leucodystrophic and paraproteinaemic forms. The correlation of these types of neuropathies to the different clinical and aetiological forms of polyneuropathies is diagrammatically shown. The connection between the electrophysiological data and the basic nerve fibre alterations of the different neuropathies is discussed.