A polyneuropathy of varying severity has been observed in association with sepsis and critical illness in 15 patients. Since clinical evaluation is often difficult, electrophysiological studies provided definitive evidence for polyneuropathy. These revealed reductions in the amplitudes of compound muscle and sensory nerve action potentials, the most marked abnormality. Near-nerve recordings confirmed such reductions for sensory fibres. Needle electromyography revealed signs of denervation of limb muscles. Phrenic nerve conduction and needle electromyographic studies of chest wall muscles suggested that the polyneuropathy partially explained difficulties in weaning patients from the ventilator, an early clinical sign. No defect in neuromuscular transmission was demonstrated, despite the use of aminoglycoside antibiotics in some patients. In those who survived the critical illness, clinical and electrophysiological improvement occurred. The 15 critically ill polyneuropathy patients were compared with 16 Guillain-Barré syndrome patients observed during the same period. The analysis showed that the two polyneuropathies are likely to be separate entities that can be distinguished in most instances by the predisposing illness, electrophysiological features and cerebrospinal fluid results.