Myositis is a rare complication of Lyme disease. In order to get information about the pathogenesis of this disorder, muscle specimens of 7 patients suffering from myositis as a manifestation of Lyme borreliosis were examined by immunohistology. Lyme spirochetes could be found in muscle biopsies of 6 patients. Infiltrates consisted mainly of macrophages and T helper/inducer cells. The T4/T8 ratio was 1.7 in the endomysium and 2.1 in the perimysium. Increased expression of MHC-I molecules by several muscle fibers was observed in 2 subjects only. No MHC-II molecules were expressed by muscle fibers. Lymphocytes expressing the interleukin-2 receptor were detected in 2 patients. Leu-15+ and Leu-11+ cells were found only to a slight extent in 2 patients. In conclusion the immunohistochemical findings in myositis due to Lyme borreliosis are different from other manifestations of this disease, and also from other forms of myositis.