Twenty-one patients with Parkinson's disease were studied because their low maintenance dosages of carbidopa-levodopa in the customary ratio of 1:10 provided less than the daily 75 mg of carbidopa believed necessary to achieve full inhibition of extracerebral dopa decarboxylation. The dosage of carbidopa was increased 2.5 times to between 75 and 150 mg daily, while the mean dosage of levodopa essentially was unchanged. The new carbidopa-levodopa ratio was 1:4. During 15 months, this treatment produced a moderate decrease in the severity of parkinsonism and a marked decrease in peripheral adverse reactions, without a significant increase in the central adverse effects of levodopa. It is concluded that increasing the dosage of carbidopa may be beneficial to patients with Parkinson's disease receiving less than 75 mg of carbidopa and 750 mg of levodopa daily.