Progressive supranuclear palsy (PSP) is the most common atypical parkinsonian syndrome and an important differential diagnosis of parkinson's disease (PD). The clinical diagnosis of PSP relies on characteristic symptoms. There is evidence of clinical subgroups within the entity of PSP interfering with making the firm diagnosis. It was the aim of the study to clarify the differences between phenotypical subtypes of PSP and PD focusing on transcallosal inhibition (TI). A systematic chart review of 67 patients supposed to have probable PSP was done in a standardized diagnostic work-up. As only complete data sets were included into further analysis, 26 PSP patients (mean age 68.6 ± 7.1 years) could be evaluated and subdivided into Richardson's syndrome (RS) (n = 15) or PSP of parkinsonian type (PSP-P) (n = 11). Fifteen PD patients served as controls. TI was evaluated by investigation of the ipsilateral silent period (iSP) with transcranial magnetic stimulation (TMS). Cognition was assessed by the Addenbrooke's cognitive examination (ACE-R). TMS revealed a significantly more severe affection of TI in RS patients as compared to PSP-P and PD patients who showed similar neurophysiological findings. 47 % of RS patient displayed an iSP loss, whereas PSP-P and PD did not. There was a significant correlation between iSP latency and ACE-R (Spearman's coefficient -0.369, P = 0.010). In conclusion, RS patients-contrary to PSP-P and PD patients-had pathological TI at least in one hemisphere indicating more severe involvement of transcallosally projecting output neurons in RS.