Polymyositis (PM) is a cell-mediated autoimmune disease. Perforin (PF), Fas ligand (FasL) and TNF-alpha are considered to be important factors in cytotoxic T lymphocyte-mediated cell injury, and several studies have established a role of lymphotoxin (LT) in T helper type 1 (Th1)-induced cell-mediated autoimmune diseases. In the present study, to determine how LT, PF and FasL are involved in the pathogenesis of PM, we used immunohistochemical staining (IHC), reverse transcription polymerase chain reaction (RT-PCR), and in situ hybridization (ISH) on muscle specimens from patients with PM, amyotrophic lateral sclerosis (ALS), myotonic dystrophy (MyD) and controls (NC). There were many mononuclear cells (MNCs) immunoreactive for LT and some for PF and FasL within the fasciculus in PM muscles. On the other hand, only few or no LT-, PF- and FasL-positive cells were detected in MyD, ALS and NC muscles. The results of mRNA expression of these three molecules with RT-PCR were consistent with those using IHC methods. The number of MNCs positive for LT with ISH was far higher in PM compared to MyD, ALS and NC (P < 0.05 or 0.01). The MNCs located in the connective tissue or in the vicinity of necrotizing or non-necrotizing muscles were mainly LT mRNA and CD4 positive, while MNCs invading the non-necrotic fibers were mainly LT mRNA and CD8 positive. Our results indicated that the expression of LT was up-regulated in PM, and LT plays an important role in muscle injury and orchestrating the inflammatory reaction in PM.