Within the last decade, Lyme borreliosis has emerged as a complex new infection whose distribution is worldwide. The disorder is caused by a recently recognized spirochete, B. burgdorferi, transmitted by ticks of the I. ricinus complex. Certain species of mice are critical in the life cycle of the spirochete, and deer appear to be crucial to the tick. Although the disorder's basic outlines are similar everywhere, there are regional variations in the causative spirochete, animal hosts, and clinical manifestations of the illness. In the United States, Lyme disease commonly begins in summer with a characteristic skin lesion, erythema migrans, accompanied by flu-like or meningitis-like symptoms. Weeks or months later, the patients may have neurologic or cardiac abnormalities, migratory musculoskeletal pain, or arthritis, and more than a year after onset, some patients have chronic joint, skin, or neurologic abnormalities. After the first several weeks of infection, almost all patients have a positive antibody response to the spirochete, and serologic determinations are currently the most practical laboratory aid in diagnosis. Treatment with appropriate antibiotics is usually curative, but longer courses of therapy are often needed later in the illness, and some patients may not respond.