Transcanial sonography (TCS) is increasingly applied in the diagnosis of Parkinson's disease (PD), but investigator bias may influence the results of examination. Blinding the sonographer to the clinical diagnosis of 42 PD patients and 35 controls, we obtained a positive predictive value of 85.7% and a negative predictive value of 82.9% in the diagnosis of PD solely by interpreting the results of TCS, indicating that TCS is a valuable additional tool in the diagnosis of PD.