A 69-year-old white man was admitted because of a clinical history of persistent cough and fever. Chest x-rays showed bilateral lung infiltrates with air bronchograms, whereas the urine antigen test resulted positive for Legionella pneumophila. The next day, he was transferred to the intensive care unit and intubated because of severe renal and respiratory distress. Neurological examination revealed distal weakness and loss of deep tendon reflexes in lower extremities. Nerve conduction studies displayed severe demyelinating sensorimotor polyneuropathy, and plasmapheresis was therefore applied with mild improvement. Few weeks after, dysphagia occurred and electrophysiologic tests showed progressive axonal involvement with spread of demyelination to the cranial nerves. The patient underwent a new plasmapheresis course and slowly reached stable clinical improvement of neurological status, which allowed him to be safely discharged. This case showed a critical onset with respiratory failure and kidney functional impairment due to L. pneumophila, subsequently disclosing Guillain-Barré syndrome.