Impaired olfaction occurs in patients with idiopathic Parkinson's disease (PD), and Lewy bodies have been found in the olfactory bulb and tract. We now confirm the latter finding and show that this presence of Lewy bodies is associated with significant neuronal loss. A quantitative study of the anterior olfactory nucleus (AON) was performed in tissue obtained postmortem from seven patients with PD and seven age-matched controls. Neuronal loss was seen in the PD bulb and tracts (p < 0.01), and a strong correlation of neuronal loss with disease duration was detected (R = -0.87). The presence of Lewy bodies was confirmed with immunocytochemical staining for ubiquitin in all the PD cases.