Mycoplasmal infections may be complicated by a wide variety of systemic syndromes, including neurological disorders. Circumstantial evidence suggests that these are immunologically-induced. A detailed study of five patients with proven rnycoplasma pneumoniae and central nervous system signs showed that immunological factors were involved and suggested that the neurological symptoms were due to a vasculopathy. Case studiesCase 1 A 44-year-old right-handed clerk was admitted to hospital having developed a right hemiparesis with non-fluent aphasia some hours before. On admission he had a single grand ma1 seizure. Five days previously he had developed a severe upper respiratory tract infection with a productive cough.Previous medical history was unremarkable except that he had had hypothyroidism for the past 3 years, for which he took replacement therapy, and he had had attacks of sinusitis several years previously. Family history was non-contributory.On examination, the patient was mute and stu-