We studied event-related desynchronization (ERD) of the 8-13 Hz rhythm (mu rhythm) of the primary somatosensory cortex (SI) caused by contra- and ipsilateral median-nerve stimulation. We used whole-head magnetoencephalography (MEG) and wavelet analysis together with our newly developed color-coded single-trial ERD display. The somatosensory stimuli suppressed mu rhythm at both contra- and ipsilateral SI, but the attenuation was clearly lateralized, being at least 20% stronger contra- than ipsilaterally. Moreover, repeated stimulation significantly reduced mu-rhythm ERD in the ipsilateral but not in the contralateral hemisphere in the course of the experiment. The observed lateralization is in agreement with the classical concept of a dominant role of the contralateral hemisphere in the processing of somatosensory information. The strong ipsilateral ERD in the beginning of the experiment may reflect the presence of non-specific arousal-like activation, which attenuates toward the end of the experiment.