OBJECTIVE To evaluate the reliability of [(123)I]meta-iodobenzylguanidine (MIBG) myocardial scintigraphy for diagnosing Parkinson's disease (PD). METHODS A series of 391 outpatients showing one or more parkinsonian-like symptoms was longitudinally followed up for accurate clinical diagnosis. MIBG scintigraphy was performed in the patients and 10 normal controls of similar age. The heart to mediastinum uptake ratio was calculated in each person, and the values were considered abnormal if they were greater than two standard deviations below the control mean. RESULTS MIBG uptake was decreased in most patients with PD (87.7%), and was seen in all advanced cases with Hohen-Yahr stage III or more; the sensitivity and specificity of scintigraphy for detecting PD were 87.7% and 37.4%, respectively. Surprisingly, over half of the patients without PD (66.5%) also exhibited low uptake, resulting in considerable overlap in the ratios between PD and the other disorders. CONCLUSIONS MIBG scintigraphy is a sensitive, but not specific, test for PD. Low MIBG uptake does not necessarily indicate PD, but is essential for diagnosing advanced PD.