Comminution of food is the composite result of selection and breakage. Selection is characterized by the chance that a food particle will at least be damaged by the teeth during a chew. For any particle size, this chance equals the ratio between the weight of damaged and broken particles and that of all initial particles. The breakage process refers to fracturing of selected particles. Since orthodontic treatment applied to patients before orthognathic surgery has an effect on the position and orientation of teeth, it may alter chewing efficiency by influencing selection and/or breakage. The effect of such orthodontic treatment on chewing efficiency was examined using eight cubes of 8.0 mm of a silicone-rubber (Optosil) as a test food for determining the number of chews required to halve the initial particle size (N(1/2)) in 12 patients. Three particle sizes (2.4, 4.8 and 8.0 mm) were used for determining selection and breakage in one-chew experiments. Orthodontic treatment had no effect on the chewing efficiency and the selection chances of all particle sizes whereas the degree of breakage was significantly (P<0.05) increased only for a particle size of 8.0 mm. These findings suggest that presurgical orthodontics has only a minor effect on food comminution.