Myositis was proven histopathologically in 4 patients (age range 36-66 years) who suffered from early or late stages of Borrelia burgdorferi infection. Muscle weakness was present in 3 patients, 1 complaining of additional myalgias. One man came to medical attention because of skin discoloration and swelling of one leg. Deep biopsy from skin, fascia and muscle revealed acrodermatitis chronica atrophicans, panniculitis, fasciitis, and myositis, respectively. Creatine kinase was slightly elevated in 3 cases and normal in one. Infiltrates were found in the perimysium and within the muscle bundles, mainly around small vessels. The infiltrates consisted of many B cells and T4 lymphocytes with fewer cytotoxic T cells, suggesting that Borrelia myositis might be due to a local immune response to unknown Borrelia antigens. Cultivation of Borrelia from muscle was not successful. Antibiotic therapy cured the myositis.