AbstractWe studied corticomotor excitability (CE) between the more and less affected sides in early Parkinson's disease (PD) patients using transcranial magnetic stimulation (TMS). Sixteen‐PD patients within the first 3 years of diagnosis were studied with single‐pulse TMS over each motor cortex with intensities from 40% to 100% stimulator output. Active motor evoked potentials (MEP) and cortical silent period durations (CSP) were recorded, fitted with sigmoid curves, summarized as maximal MEP/CSP, maximal MEP/CSP slope, and intensity where MEP/CSP is half‐maximal (MEP/CSP‐Int50), and correlated with Unified Parkinson's Disease Rating Scale scores (UPDRS). On the more affected side, higher (worse) UPDRS scores were correlated with shorter maximal CSP (r = −0.51, P = 0.046). On the less affected side, higher UPDRS scores were correlated with higher MEP‐Int50 (r = 0.51, P = 0.043) and CSP‐Int50 (r = 0.54, P = 0.029). For the less affected side, altered CE, as indexed by higher MEP or CSP‐Int50 intensities, may contribute to early clinical symptoms. On the more affected side, increases in CE, indexed by shorter CSP, may account for a greater proportion of PD symptoms. These findings are consistent with an evolution of neurophysiologic correlates in early PD patients from a less to more symptomatic state. © 2007 Movement Disorder Society