Olfactory loss is a prominent symptom in idiopathic Parkinson's disease (IPD). Experiment 1 re-investigated the diagnostic value of psychophysical testing in the differentiation between idiopathic Parkinson disease (IPD) from non-IPD; 50 consecutive PS patients participated. In Experiment 2 five de-novo patients received 3 olfactory tests spread over a period of appoximately one year. Nineteen IPD patients were anosmic, and 18 were hyposmic. All but one patient with MSA and PSP had mild/moderate hyposmia. Normosmia was found in CBD/misdiagnosed PS/psychogenic movement disorder. In Experiment 2, one of the de-novo patients was normosmic, 3 hyposmic, and 1 anosmic. Follow up investigations indicated decreased olfactory function in 3 patients while it improved in one. The normosmic patient retained olfactory abilities. This patient failed to respond to pharmacological treatment. In summary, olfactory tests differentiate IPD from non-IPD. Furthermore, tests of olfactory function may also be of interest in investigations related to treatment of PS.