Myositis is a rare manifestation of Lyme disease of unknown pathogenesis. This study describes the course of disease in eight patients with Lyme disease, aged 37-70 years, all of whom were suffering from histologically proven myositis. The clinical, electrophysiological, and myopathological findings are reported. One patient showed signs and symptoms of myositis of all limbs. In six patients myositis was localized in the vicinity of skin lesions, arthritis or neuropathy caused by Borrelia burgdorferi. In another patient suffering from pronounced muscle weakness of the legs and cardiac arrest, inflammation of the myocardium, the conducting system and skeletal muscles was revealed at autopsy. Muscle biopsy revealed lymphoplasmocellular infiltrates combined with few fibre degenerations in three patients. The lymphoplasmocellular infiltrates were found predominantly in the vicinity of small vessels. Several spirochetes were stained in six of seven muscle biopsy samples by means of the immunogold-silver technique. Culturing of B. Burgdorferi from the muscle biopsy samples was, however, unsuccessful. Antibiotic treatment succeeded in curing the myositis in four of six patients. In one patients signs and symptoms improved. One patient died from cardiac arrest caused by myocarditis and Guillain-Barré syndrome. The outcome is unknown in one patient. Clinical and myopathological findings indicate that Lyme myositis can be caused either by local spreading of B. burgdorferi or an unknown antigen or toxin from adjacent tissues or haematogenously.