SUMMARY A study of the clinical, electrophysiological and biochemical features of 16 patients with legionellosis has been performed. Evidence of central and peripheral nervous system involvement has been found in the majority of patients. This is characterised by confusion out of keeping with the degree of toxic or metabolic upset, signs of anterior midline cerebellar dysfunction, grossly elevated creatinine kinase of skeletal muscle origin, and a subclinical peripheral neuropathy.Since the epidemic of severe pneumonia at an American Legion Convention in 1976' the term legionnaires' disease has been associated with a serious respiratory system disorder. Infection with the causative pathogen, Legionella pneumophila is now more appropriately termed legionellosis to emphasise the multisystem features including pneumonia, abdominal distension and paralytic ileus, renal impairment, shock, icterus, coagulopathy and neurological upset.'-3 In view of the difficulty of isolating the causative bacillus and the current dependence on serological testing for positive diagnosis the clinical features are crucial. This is important in view of the limited therapeutic options and high mortality in untreated cases.45The neurological aspects have been described by several workers in case reports,2 5-l1 but no comprehensive study has been made of these features. We report our findings of 16 cases of legionellosis which have been examined and investigated neurologically, out of a total of 28 which have passed through the infectious diseases and chest medicine units of Ruchill Hospital, Glasgow. We have briefly communicated our findings before'617 and two of the cases reported have also been described previously by Lees and Tyrrell.