Of 25 patients with Behçet's disease seen in five years at the Mayo Clinic, seven had central nervous system involvement. The mean interval from onset of Behçet's disease to central nervous system involvement was 1.3 years, and the mean period of observation thereafter was 3.8 years. All patients had headache and fever during or preceding exacerbations of the central nervous system disease, and all had cerebrospinal fluid pleocytosis (white cell counts ranged from 6 to 490/mm3) with predominant lymphocytosis. The mean cerebrospinal fluid protein level was 55 mg/dl, and gamma globulin was less than 15 per cent in six patients. Results of cerebrospinal fluid, lesional and serologic studies for bacterial, fungal and viral agents were negative. Clinical findings included corticospinal tract disease (five patients), cerebellar ataxia (four patients), pseudobulbar palsy (three patients) and transient ocular palsies (three patients). All these occurred in concert with the systemic phases, especially aphthosis. Corticosteroid therapy was used in six patients, cyclophosphamide therapy in two and azathioprine therapy in two. The neurologic manifestations tended to recur when dosages of the drugs were lowered and established central nervous system damage could not be reversed. The case histories suggest that steroids, when used promptly and in sufficient dosage (up to 60 mg prednisone), are effective in reducing or preventing progression of central nervous system disease. Three patients died, one from Pneumocystis carinii infection, one from neurologic disease and another from presumed myocardial infarction. The central nervous system involvement in Behçet's disease should be diagnosed earlier, and it can be if attention is given to other systemic criteria, that is, aphthous stomatitis, aphthous genital lesions, cutaneous vasculitis, uveitis and synovitis.