Background: Critical illness myopathy (CIM) and polyneuropathy (CIP), alone or in combination (CIP/CIM), are frequent complications in patients in the intensive care unit (ICU). There is no evidence that differentiating between CIP and CIM has any impact on patient prognosis. Methods: 1-year prospective cohort study of patients developing CIP, CIM or combined CIP and CIM during ICU stay. Results: 28 out of 92 (30.4%) patients developed electrophysiological signs of CIP and/or CIM during their ICU stay, which persisted in 18 patients at ICU discharge. At hospital discharge, diagnoses in the 15 survivors were CIM in six cases, CIP in four, combined CIP and CIM in three and undetermined in two uncooperative patients. During the 1-year follow-up of six patients with CIM, one patient died and five recovered completely within 3 (three patients) to 6 (two patients) months. Of three patients with CIP/CIM, one died, one recovered and one with residual CIP remained tetraplegic. Of four patients with CIP, one recovered, two had persisting muscle weakness and one remained tetraparetic. Conclusion: CIM has a better prognosis than CIP. Differential diagnosis is important to predict long-term outcome in ICU patients.