In 13 elderly patients, 12 of whom had Parkinson's disease, visual hallucinations and delirium developed as a side effect of amantadine hydrochloride (Symmetrel) therapy. The symptoms promptly disappearred when amantadine was discontinued. Thereafter, each parkinsonian patient was treated satisfactorily with levo-dopa. Treatment with a combination of amantadine and an anticholinergic agent increases the likelihood of delirium because of the hazard of retention of urine. Although amantadine is effective in the treatment of Parkinson's disease in the elderly, the incidence of delirium as a complication seems higher in this age group.