Lyme disease is a tick-borne illness that has been reported from three regions in the United States--the Northeast, Midwest, and West--which correspond to the distribution of the recognized vectors of the disease, Ixodes dammini and Ixodes pacificus. In 1982, a surveillance system designed to define the morbidity and geographic distribution for Lyme disease by using a clinical case definition received information on 491 definite cases and 38 probable cases. Of the definite cases, 489 were acquired in endemic areas of the Northeast or Midwest; one case was acquired in Utah and one in western Pennsylvania, two areas where the illness had not been previously reported. Three states that previously had not reported cases of Lyme disease (Kentucky, Indiana, Montana) reported probable cases. In 37% of the definite cases, the patients had neurological symptoms (most commonly reported was headache with stiff neck, suggestive of meningitis), 10% cardiac symptoms (most commonly reported was palpitations), and 54% arthritic symptoms (most commonly reported in large joints). The occurrence of Lyme disease in areas outside the currently recognized endemic regions, as well as the recent description of Amblyomma americanum as a probable vector, suggest that additional vectors may be described in the future.