Although unilateral lesion studies concerning the opercular part of primary motor cortex report clinically severe motor deficits (e.g. anarthria, masticatory paralysis), functional lateralization of this area has not yet been addressed in neuroimaging studies. Using BOLD-FMRI, this study provides the first quantitative evaluation of a possible cortical lateralization of symmetric chin movements (rhythmic contraction of masticatory muscles) in right-handed healthy subjects and presurgical patients suffering tumorous lesions in the opercular primary motor cortex. Data were analyzed according to "activation volume" and "activation intensity". At group level, results showed a strong left-hemispheric dominance for chin movements in the group of healthy subjects. In contrast, patients indicated dominance of the healthy hemisphere. Here, a clinically relevant dissociation was found between "activation volume" and "activation intensity": Although "activation volume" may be clearly lateralized to the healthy hemisphere, "activation intensity" may indicate residual functionally important tissue close to the pathological tissue. In these cases, consideration of BOLD-FMRI maps with the exclusive focus on "activation volume" may lead to erroneous presurgical conclusions. We conclude that comprehensive analyses of presurgical fMRI data may help to avoid sustained postoperative motor deficits and dysarthria in patients with lesions in the opercular part of primary motor cortex.