Erythema chronicum migrans (ECM), the skin lesion characteristic of Lyme disease, was first described in Sweden in 1909; subsequently, cases of ECM have been reported from at least 19 countries on three continents. In Europe cases have occurred within the range of Ixodes ricinus ticks, the recognized vector of ECM in Europe, although one case outside this range has been ascribed to mosquito bites. In 1970 the first case of ECM acquired in the United States was reported, and in 1977, the full symptom complex now called Lyme disease was described. In the United States three geographic areas endemic for Lyme disease are recognized: the coastal areas of the Northeast; Minnesota and Wisconsin in the Midwest; and parts of California, Oregon, and western Nevada in the West. These areas correspond to the recognized distribution of Ixodes dammini in the Northeast and Midwest and Ixodes pacificus in the West. Isolated cases of illness compatible with Lyme disease have, however, been reported from other parts of the United States; this suggests that cases may be more widely distributed than is currently recognized and that other vectors may be involved. Supporting this suggestion is the description in 1982 of Lyme disease in Australia, where none of the currently recognized vectors are known to exist, and the report in 1984 that Ambylomma americanum ticks can harbor the spirochete that causes Lyme disease. The recognition of cases acquired in widely separated parts of the world involving multiple vectors suggests that cases may in the future be diagnosed in additional areas.