Cardiac sympathetic dysfunction was investigated using 123I-metaiodobenzylguanidine (MIBG) myocardial scintigraphy in 20 patients with drug induced parkinsonism (DIP). The mean heart to mediastinum ratio was significantly greater in patients with DIP than in those with Parkinson's disease (mean (SD): 2.07 (0.39) v 1.28 (0.15), p<0.001). MIBG uptake was not different between the DIP patients and controls. Two DIP patients whose MIBG uptake was significantly reduced showed persistent parkinsonism and responded dramatically to levodopa.