In the absence of clinical signs, elevated values of the cardiac isoforms of troponin T (cTnT) and I (cTnI) can be found in the serum samples of some patients with skeletal muscle myopathies; the cause is unclear. We studied the messenger RNA (mRNA) expression of cTnT and cTnI in the skeletal muscles of 24 patients with histologically proven myopathies and in 18 patients in whom a myopathy could be excluded. For cTnT- and cTnI-mRNA determination, we designed specific primer pairs for nested polymerase chain reaction. After amplification, the products were digested with 2 restriction enzymes and visualized. We found cTnT mRNA in 7 skeletal muscle biopsy specimens (6 patients with Duchenne muscular dystrophy, 1 patient with a primary sarcoglycanopathy) and cTnI mRNA in 6 (5 with Duchenne muscular dystrophy, 1 patient with a histologically negative biopsy). The mRNA of the cardiac isoforms, cTnT and cTnI, is expressed in the skeletal muscles of patients with Duchenne muscular dystrophy, but also in some other myopathies. Further studies are needed to show whether the mRNA is translated into the protein, but serum levels of cTnT and cTnI in patients with Duchenne muscular dystrophy would seem to indicate this.