The aim of this study was to determine the amount of myosin heavy chain (MyHC) proteins and MyHC mRNA in muscles of patients with different positions of the mandible. Ten adult patients for orthognathic surgery were divided into two groups: distal and mesial malocclusion. The mRNA expression of two MyHC isoforms of the anterior and posterior part of the right and left side of the human masseter muscle was analysed with a competitive RT-PCR assay. An exogenous template that includes oligonucleotide sequences specific for sarcomeric MyHC isoforms (1 and 2x) was constructed and utilized as competitor. Different isoforms of the MyHC protein were identified by Western blot analysis. In the total mRNA pool of the masseter muscle, the MyHC 1 mRNA level was 25.5 ? 7.6% and the MyHC 2x mRNA was 2.5 ? 1.2%. The anterior part of the masseter muscle from patients with distal occlusion contained more type 1 and 2x MyHC mRNA, as compared to patients with mesial occlusion (P < 0.05). No difference in the protein distribution was observed. The differences in mRNA expression may be caused by the enforced stress of the masticatory muscle in distal occlusion because of the disadvantageous pivot.