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Multiple System Atrophy (MSA) and idiopathic Parkinson's disease (PD) can be difficult to distinguish. There is an ongoing debate about the diagnostic value of the growth-hormone response to clonidine (CGH-test) in PD and MSA. We investigated whether the CGH-test can identify individual patients in the early stages of PD (n = 21) and Striatonigral Degeneration (SND, n = 11), a particular variety of MSA. Patients were diagnosed on the basis of clinical criteria and IBZM-SPECT. Clonidine induced a greater total serum growth-hormone production in PD than in SND (p = 0.01). However, taking the difference in prevalence of PD and SND into account, and because of the low likelihood ratios of the test, an increase of GH after clonidine increases the pre-test probability for PD by about only 5 %, while an absent response of GH also increases the pre-test probability for SND by about 5 %. We conclude that the CGH-test discriminates between groups of patients with PD and SND, but has little practical diagnostic value for identifying individual patients. [ABSTRACT FROM AUTHOR]