Dopa-responsive dystonia (DRD) is a genetic disorder characterized by childhood onset dystonia, dominant inheritance, diurnal symptoms fluctuation and positive levodopa response. Adult-onset DRD is frequently combined with parkinsonism and can be mistaken with young onset Parkinson's disease (YOPD). Both conditions are caused by dopamine deficiency, due to nigral cells' loss in YOPD, and due to enzymatic defects in dopamine synthesis in DRD. Single photon emission tomography (SPET) with (123)I-N--fluoropropyl-2b-carbomethoxy-3b-(4-iodophenyl) nortropane ((123)I-FP-CIT)-DaTSCAN is a sensitive neuroimaging method for the assessment of nigrostriatal dopaminergic system integrity and degeneration. Our aim was to evaluate the usefulness of (123)I-FP-CIT( DaTSCAN) SPET in the differential diagnosis of DRD and YOPD in clinical practice. Brain SPET with (123)I-FP-CIT was performed in 13 patients (7 males, 6 females), age 20-58 years, with mean age of onset of their disease, 29 years, eleven patients with early onset parkinsonian symptoms and 2 with genetically proved DRD. The images were evaluated by visual and semiquantitative analyses (ROI). The ratio of specific-striatal to non specific-occipital binding was calculated. Ten out of 11 patients with YOPD had decreased accumulation of DaTSCAN in striatum, especially in putamen, that is typical findings for Parkinson's disease. In three patients DaTSCAN was normal with symmetric tracer uptake in both striata, caudate nucleus and putamen and the diagnosis of DRD was suspected. Two patients with initial dystonic symptoms and genetically proved DRD had normal DaTSCAN. In one patient after normal DaTSCAN findings the initial diagnosis of YOPD was changed to the diagnosis of DRD. Region of interest (ROI) analyses have shown significantly lower(123)I-FP-CIT binding ratios in YOPD than in DRD in all 3 regions of interest: striatum (1.95±0.32) vs (2.76±0.10) P<0.001, putamen (1.76±0.25) vs (2.84±0.14) P<0.0001 and caudate nucleus (2.37±0.51) vs (3.27±0.14) P<0.01. In conclusion, our results indicate that DaTSCAN is an objective neuroimaging method able to distinguisch neurodegenerative disease YOPD from DRD and clarify a clinical dilemma, which is important for the treatment, prognosis and genetic counseling of patients and their families.