To evaluate the spectrum and time profile of electrophysiological parameters in the detection of neuromuscular involvement in critically ill patients and establish their correlation with biopsy findings. Prospective clinical and neurophysiological study. One general and one neurological intensive care unit in a university hospital. Forty-six critically ill patients with failure of at least two organ systems were enrolled and completed the 1-month follow up. Detailed clinical and electrophysiological evaluation including direct muscle stimulation was performed in all cases on entry and at the end of the follow-up. Muscle biopsy was performed in 11, and sural nerve biopsy in 5, cases. Electrophysiological signs of new or progressing neuromuscular involvement at the end of the first month were detected in 26 patients (56%) and could be classified into three groups: "pure motor syndrome" (12 cases), combined motor syndrome and sensory polyneuropathy (13 cases) and isolated sensory polyneuropathy (1 case). Direct muscle stimulation showed decreased muscle membrane excitability in 11 of these abnormal cases. Muscle biopsy disclosed various myopathic abnormalities in all 11 cases examined with motor syndrome, in 7 of them in association with denervation/re-innervation changes. Electrophysiological and histological examinations showed significant overlapping of several pathogenic components of neuromuscular involvement in critically ill patients, namely decreased muscle excitability, myopathy, axonal motor neuropathy and sensory neuropathy. The characterisation of the electrophysiological components of a complex polyneuromyopathy is preferred to the strict categorisation of abnormalities into critical illness myopathy and polyneuropathy.