The diagnosis of Parkinson's disease (PD) is currently based on the clinical evaluation of extrapyramidal signs with a considerable error rate. The identification of specific markers might allow PD diagnosis before the onset of classical motor symptoms. By two-dimensional electrophoresis we identified proteome alterations in T-lymphocytes of 17 control subjects and 15 PD patients. The observed changes were used to build predictive models that were verified by the leave-one-out cross-validation. We further built two functions able to stage the subjects. We chose to verify by Western blotting the identity of spots corresponding to β-fibrinogen and transaldolase, two recurrent proteins in six out of 20 spots. β-Fibrinogen levels are lowered in PD patients, whereas a heavy transaldolase set of isoforms was more abundant. Eventually, we identified a list of seven proteins showing different levels in early-onset with respect to late-onset PD patients.