Lyme disease, caused by the spirochete Borrelia burgdorferi, has classically been divided into three stages: erythema migrans; neurological or cardiac involvement; and arthritis. Rather than defining a set disease pattern, however, one should, more logically, conceptualize a progressive infection that may be localized or disseminated, acute or chronic. Erythema migrans, the earliest and most easily recognized manifestation of B burgdorferi infection, is an expanding annular erythematous skin lesion with a central clearing that develops soon after the bite of an infected ixodes tick. Musculoskeletal manifestations are common, with approximately one-half of untreated individuals developing arthritis. Of these, only 10% have chronic arthritis. Invasion of the central nervous system occurs as the infection disseminates hematogenously, with encephalitis, myelitis and meningopolyneuritis being the most severe results. Acute cardiac involvement is recognized in up to 8% of adult patients, and less often in children. Early antibiotic treatment of the infection is highly effective.