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Fourteen radiation-induced brachial plexus neuropathies in 12 patients suffering from cancer were studied. Neurophysiological evaluation showed a diffuse neurogenic lesion with muscular denervation signs associated with motor and sensory nerve conduction impairment of axonal type in the distal segments of the arm. Somatosensory evoked potentials were frequently abnormal, with absence of N9 in 9 out of the 10 extremities explored. The most characteristic findings were, however, the presence of fasciculation potentials--single and grouped--and myokymic discharges in 78.5% of cases (11 out of 14 plexuses), and a motor nerve conduction block on proximal stimulation, at the supraclavicular as well as cervical spine levels, in all of the cases. Both phenomena showed a high correlation when analyzed in the same neuromuscular territory. The 5 muscles with no voluntary activity and complete--or nearly complete--motor nerve conduction block were the ones with the most intense ectopic activities. The conduction blocks were present after long periods of illness in all cases and, in 2 of the cases, they persisted in successive explorations at intervals of 9 months and 2 years respectively. These data would support a probable cause-effect relationship between a persistent and prolonged motor nerve conduction block and the presence of fasciculation-myokymic type activities. One could even postulate that the infrequent neuropathies, in which both findings have been described as relevant features, have a similar physiopathological mechanism.