This study presents the main clinical and electrophysiological features of patients with Charcot-Marie-Tooth (CMT) disease or hereditary motor and sensory neuropathy (HMSN) in Northern Finland diagnosed during the period 1969-1983. The group consists of 40 patients aged between 5 and 74 years. A positive family history suggesting a dominant inheritance was seen in 23 patients--the rest were classified as sporadic. According to median or sural nerve conduction velocities the disease was graded as demyelinating (HMSN type I) and neuronal (HMSN type II). A clear tendency towards an earlier onset was seen in type I cases compared to those in type II. Both clinically and neurophysiologically the lower arms were more severely affected in type I than in type II. Muscular atrophy of the legs occurred more often in type I than in type II. Electromyographic abnormalities were characteristically neurogenic, though in some advanced cases small motor unit potentials could also be seen. In addition 2 cases originally diagnosed as CMT proved to be distal myopathy on electromyography (EMG). According to electrodiagnostic criteria, patients in the same family could belong to different subgroups of CMT suggesting that the present use of conduction velocities as the discriminating factor between the groups may be unsound.