Lyme disease is increasingly being reported throughout the United States and many parts of the world. Borrelia burgdorferi, the etiologic agent of Lyme disease, is a spirochete that, not unlike the treponema of syphilis, can cause a spectrum of disease from the initial skin lesion, through widely varied symptoms and signs, to chronic neurologic and arthritic disability. The borrelial spirochete and Lyme disease are the subject of this review. A subsequent article will review other definite and possible cutaneous manifestations of borreliosis.