Following the original description of erythema chronicum migrans (ECM) in Sweden in 1909, ECM became widely recognized in Europe. The first reported case of ECM acquired in the United States occurred in 1969, and in 1975 the full symptom complex now known as Lyme disease was recognized. In 1981, cases of Lyme disease were recognized in yet a third continent, Australia and, to date, cases acquired in at least 19 countries have been reported. Beginning with the original case reported in Sweden, clinical observations suggested that Ixodes ricinus ticks were a vector for ECM in Europe and the distribution of cases in Europe corresponds to the distribution of this tick, although one case outside this range has been reported following mosquito bites. Through similar observations, I. dammini and I. pacificus ticks have been established as vectors in the United States. In Australia, a vector has not been established, and none of the recognized vectors of Lyme disease occur there. The reporting of cases of Lyme disease from widely separated parts of the world involving multiple vectors suggests the disease may, in the future, be recognized in additional areas.