Disrupted interhemispheric connectivity (IHC) is being increasingly recognized as a robust feature of a number of neurological diseases, including Parkinson's Disease (PD). We investigated whether or not Galvanic Vestibular Stimulation (GVS) - a proposed treatment for PD - affected IHC in 11 PD subjects (off medication). In order to avoid the confound of stimulation artifact disrupting the EEG, we investigated the immediate period (52 sec) after stimulation (72 sec) with noisy 1/f-type GVS stimuli. Partial Least Squares (PLS) regression was used to determine the subject-specific linear combination of EEG electrodes that maximized covariance between hemispheres and then computed coherence between the dominant PLS components. PD subjects had increased IHC <;~10 Hz and decreased IHC >13 Hz compared to 11 healthy, age-matched controls. After GVS in PD subjects, a significant increase in IHC was detected, especially above 30 Hz. We suggest that GVS may partially exert its beneficial effects in PD by "normalizing" impaired IHC.