Echophenomena in Gilles de la Tourette syndrome (GTS) may relate to deficient processing of observed biological movements. This would be reflected in altered effects of movement observation on motor responses in these patients. We studied reaction times in 11 unmedicated GTS patients without psychiatric comorbidity and healthy subjects. In experiment 1, participants imitated single biological finger movement stimuli or nonbiological dot movement stimuli immediately. In experiment 2, participants responded to a tone while viewing biological or nonbiological movement stimuli that were either compatible (identical) or incompatible (different) with their response. In experiment 1, both patients and healthy subjects responded faster to single biological than to nonbiological stimuli. In experiment 2, biological stimuli caused a larger compatibility-effect in responses than nonbiological stimuli in both groups, provided stimulus presentation and response initiation coincided. Healthy subjects responded faster to compatible biological than nonbiological stimuli. In contrast, GTS patients responded slower to incompatible biological than nonbiological stimuli. Patients' mean reaction time in experiment 2 correlated with phonic tic-frequency. Motor facilitation by observing biological movements appears to rely on concomitance of stimuli and responses in GTS patients and healthy individuals. Differing behavioral effects of movement observation in GTS might reflect altered activation of an action observation-execution matching system. To avoid unwanted movements GTS patients probably have to inhibit motor activation induced by observed movement automatically. Thus, movement stimuli may facilitate similar motor responses less but interfere more with different responses in these patients. (c) 2010 Movement Disorder Society.