BACKGROUND: Olfactory dysfunction is common in patients with Parkinsons disease (PD) and may precede the development of parkinsonian motor symptoms. Cardiac sympathetic denervation, which can be visualized by a cardiac (123)I-metaiodobenzylguanidine (MIBG) scan, is common in patients with PD. In this study, we evaluated olfactory functions in patients with IPD and MSA, and investigated an association between olfaction and cardiac 123I-MIBG uptake in these patients. METHODS: We prospectively enrolled 26 patients with PD, 19 patients with MSA, and 18 healthy controls. Olfactory function was evaluated with a 12-Item Cross-Cultural Smell Identification Test (CC-SIT) and Butanol threshold. 123I-MIBG (111 mBq) was injected intravenously into each subject, and cardiac uptake was imaged 3 hours later. The regions of interest were the whole heart and the mediastinum of the front image, and the ratio of 123I-MIBG uptake in the heart to that in the mediastinum (H/M ratio) was calculated. The clinical stages of parkinsonism were assessed according to the classification of Hoehn and Yahr (H&Y) and the Unified PD Rating Scale (UPDRS). RESULTS: The mean CC-SIT score in patients with PD was 4.4+/-2.2, which was significantly lower than that in patients with MSA (6.7+/-2.0) or in controls (7.3+/-2.6). There was a significant positive correlation between cardiac 123I-MIBG uptake and the CC-SIT score in patients with PD (r=0.56, p=0.003). Neither the CC-SIT score nor cardiac 123I-MIBG uptake were significantly correlated with the disease duration, the H&Y stage or motor UPDRS score. In the patients with MSA, the CC-SIT and cardiac 123I-MIBG uptake did not show a significant correlation with age (r=0.01 and r=0.11, each p>0.05), and they were not significantly correlated with each other (r=0.01, p>0.05). CONCLUSIONS: Our data suggest that the functional loss of the olfactory and cardiac sympathetic systems is closely coupled in PD.