Five patients developed a severe motor and sensory polyneuropathy at the peak of critical illness (sepsis and multiorgan dysfunction complicating a variety of primary illnesses). Difficulties in weaning from the ventilator as the critical illness subsided and the development of flaccid and areflexic limbs were early clinical signs. However, electrophysiological studies, especially needle electrode examination of skeletal muscle, provided the definite evidence of polyneuropathy. The cause is uncertain, but the electrophysiological and morphological features indicate a primary axonal polyneuropathy with sparing of the central nervous system. Nutritional factors may have played a role, since the polyneuropathy improved in all five patients after total parenteral nutrition had been started, including the three patients who later died of unrelated causes. The features allow diagnosis during life, and encourage continued intensive management since recovery from the polyneuropathy may occur.