Lyme disease is a recently described clinical entity with cutaneous, neurologic, articular and cardiac manifestations. Since the original description of the disease in 1977, more than 500 cases have been reported. Although the vast majority of patients have been from the area near Lyme, Connecticut, we have seen four patients from northern California with various aspects of Lyme disease. This diagnosis should be considered in patients who have traveled to tick regions and who have a distinctive skin lesion (erythema chronicum migrans). Clinical and epidemiologic evidence suggests the disease is transmitted by a tick, lxodes dammini in the eastern United States, lxodes pacificus in the West and lxodes ricinus in Europe. The etiologic agent, a new spirochete, may have been recently discovered.