Unilateral chewing habit is common in dentate populations. It may initiate a hypertrophy process in the lateral pterygoid muscles compared with the nonused sides. We hypothesized that the lateral pterygoid muscles of the affected sides would be larger in volume than those of the nonaffected sides in subjects with unilateral chewing habit. Because analysis of medical images in accordance with the rules of modem stereology enables precise unbiased estimates of muscle volume, and magnetic resonance imaging (MRI) is a criterion standard as an imaging modality for the analysis of muscles because of its excellent soft-tissue resolution, we estimated lateral pterygoid muscle volume both at fight and left sides using stereologic techniques (Cavalieri principle) by means of MRI. The MRI-estimated volumes of both the inferior and the superior heads of the lateral pterygoid muscle at the chewing side were significantly greater than the opposite side when compared. This finding may assist in the interpretation of lateral pterygoid muscle and temporomandibular joint-related problems in maxillofacial surgery clinics. This investigation has also demonstrated that systematically sampled MRI slices through the lateral pterygoid muscle can be used to obtain unbiased estimates.