A double-blind cross-over study was carried out in 54 patients with Parkinson's disease to evaluate the efficacy of amantadine hydrochloride as compared to a lactose placebo in the management of this illness. Amantadine proved to be a useful and safe addition to the armamentarium when given in daily doses of 200 mg. Forty-eight per cent of patients experienced moderate to good results while 31% showed no measurable improvement. The quality of the improvement was inferior to that obtained with levodopa, but the side effects were fewer. The study could not demonstrate a useful synergistic action between the two drugs, nor could the response to amantadine be used to predict that with levodopa. On the other hand, the addition of amantadine was useful in a few instances where optimal therapeutic doses of levodopa could not be given because of side effects. The mechanism of action of amantadine is still conjectural, but there is strong evidence to indicate some interaction with central dopamine metabolism.