The early diagnosis of Parkinson's disease (PD) represents a challenge, and novel accurate biomarkers are therefore needed urgently. Detection of phosphorylated alpha-synuclein (p-alpha-syn) in skin nerve fibres has shown promise as such a marker. However, its accuracy for the identification of PD among patients with early signs of parkinsonism has not been explored thoroughly. In this blinded, multicentre, prospective and follow-up study, 151 patients diagnosed with early-stage parkinsonism (<18 months duration) were enrolled at three tertiary movement disorder centres. Clinical scales were performed, and initial diagnoses were reassessed at 18- and 46-month follow-up visits. Skin biopsy, with analysis for neuronal and glial p-alpha-syn deposits, and olfactory testing were performed at the baseline visit and repeated in 44 patients at the 18-month visit. After the follow-up period, a final diagnosis was reached in 140 patients: PD (n = 101; 67% of all screened patients), tauopathies (n = 22; 15%), multiple system atrophy (n = 5; 3%), vascular parkinsonism (n = 4; 3%), essential tremor (n = 3; 2%), dystonic tremor (n = 2; 1%) and no neurological illness (n = 3, 2%). Eleven patients did not fit any clinical criteria and were therefore classified as undefined. Baseline skin intraneural p-alpha-syn showed a robust diagnostic accuracy (81% sensitivity and 100% specificity) for identifying PD. Importantly, in 30 of 44 patients not diagnosed with PD until the follow-up, intraneural p-alpha-syn was already positive at baseline. Moreover, the analyses showed a large degree of consistency over time, in that the same results at baseline and follow-up were obtained in 42 (96%) of the patients tested. Finally, although olfactory testing at baseline showed a more abnormal score in PD compared with the other groups, its predictive accuracy was more modest (72%). In conclusion, given its high sensitivity, specificity and reproducibility, skin intraneural p-alpha-syn could become a valuable tool for diagnosing PD at early stages, potentially years before the diagnostic criteria are met, and for differentiating PD from atypical parkinsonism. In contrast, olfactory function, although more impaired in PD than in non-PD patients, seems to have only a more limited diagnostic accuracy.