Sixteen parkinsonians with acquired drug-resistant "off" periods without dyskinesia were placed on a diet in which virtually all protein was concentrated in the evening meal. Restoration of sensitivity to levodopa resulted in 88%. Ten patients (62%) have continued to comply with the diet for 7 months (mean). Two patients were studied in detail. Immobility correlated with elevated plasma levels of large neutral amino acids (LNAA), normality with low LNAA.