To explore the nature of the olfactory dysfunction associated with Parkinson's disease (PD), 81 PD patients who scored well on a cognitive screening test were administered the 40-odorant University of Pennsylvania Smell Identification Test; 38 were additionally given a forced-choice phenylethyl alcohol odor detection threshold test. Clinical ratings of 11 neurologic symptoms (three bilateral) were obtained at the time of testing, and odor identification was retested in 24 patients at intervals ranging from 5 to 39 months. Relative to matched controls, the PD patients exhibited consistent and marked decrements on both types of olfactory tests (ps less than 0.0001). The odor identification deficit was not restricted to any subset of odorants and did not evidence longitudinal change. A factor analysis of the intercorrelations among the variables yielded six easily interpretable factors: general motor, oral motor, olfactory function, cognitive function, tremor, and gender. Olfactory test scores were independent of all other measures, including disease stage and duration. Seventy-two percent of the PD patients were unaware of a smell disorder before testing; those who were aware had significantly lower test scores. A statistical comparison of PD patients' olfactory test scores to those obtained from Alzheimer's disease patients found the olfactory disorders of these diseases to be indistinguishable. The data support the hypothesis that the olfactory deficit of PD is a general and stable one which likely occurs early in the disease process.