Olfactory function is diminished in patients with idiopathic Parkinson disease (IPD). Because previous work almost exclusively relied upon cross-sectional studies, the present investigation aimed to address the correlation between olfactory loss and duration of disease within the context of a longitudinal study, accompanying well-diagnosed patients over an average period of 4.4 years. A group of 27 IPD patients was examined (5 women, 22 men; age range 27-64 years; duration of disease: 0 to 19 years). Psychophysical olfactory testing was performed after 3-6 years (mean 4.4 years) using the "Sniffin' Sticks" test battery which consists of subtests for odor thresholds, odor discrimination, and odor identification. The study yielded the following major results: (1) olfactory function in IPD patients changes in an unpredictable manner, (2) especially when considering results from the second session relatively few IPD patients were completely anosmic; none of the patients, however, were normosmic. One possible explanation for these findings may lie in the hypothesis based on results by Huisman et al. (2004) who reported an increase of dopaminergic neurons in the olfactory bulb in IPD patients. In this scenario, olfactory loss seen early in the disease may be based on an incomplete inhibition of olfactory input at the level of the olfactory bulb.