Inclusion body myositis (IBM) is a chronic inflammatory myopathy. The muscle histology is characterized by infiltration of T cells, which invade and apparently destroy muscle fibres. This study was performed to investigate whether predominant clones of muscle-infiltrating T cells are identical in different muscles and whether they persist over time in IBM. By reverse transcriptase-polymerase chain reaction, 25 T-cell receptor (TCR) variable beta (Vbeta) chain families and the complementarity-determining region 3 (CDR3) of the TCR were analysed in two different muscle biopsies of four patients with IBM. In two of the patients, the muscle biopsies were obtained from different muscles at one time point, whereas in two patients, the second biopsy was obtained 9 years after the first biopsy. T cells expressing predominant Vbeta families were analysed for clonality by fragment length analysis of the CDR3. Predominant Vbeta families were analysed by DNA sequencing to identify identical clones. Immunohistochemical staining of Vbeta families was performed to study the distribution of T cells expressing identified predominant Vbeta families. The muscle-infiltrating lymphocytes showed restricted expression of TCR Vbeta families. DNA sequencing proved that clonally expanded T cells were identical in different muscles and persisted 9 years after the first biopsy. Immunohistochemical analysis with Vbeta family-specific antibodies demonstrated the endomysial localization of these T cells in inflammatory cell infiltrates. Our results show that in IBM there is clonal restriction of TCR expression in muscle-infiltrating lymphocytes. Identical T-cell clones predominate in different muscles, and these clones persist for many years. These results indicate an important, continuous, antigen-driven inflammatory reaction in IBM.