Objectives: Orthognathic surgery leads to changed jaw position and force vector of mastication to which the muscles must adapt. The aim of the present study was to determine the relative expression of myosin heavy chain (MyHC) messenger RNA (mRNA) isoforms in different types of human masseter muscle fiber under consideration of change in the number of occlusal contacts before and 6 months after surgery. Study design: Muscle biopsies were taken from the anterior and posterior parts of both sides in 30 patients with prognathic and retrognathic mandibles. Specific mRNA MyHC analysis was made by real-time polymerase chain reaction to quantify the isoforms I, IIa, and IId/x. Results: There was a shift in the relative content from type I (46% before, 37% after) to type IIa (29% before, 42% after). This shift correlates with number of teeth in occlusion. Conclusions: Correlation between isoform shift and number of teeth in occlusion indicates higher mastication force which stabalizes the treatment result.