Lyme disease has emerged as the leading vector-borne disease in the United States. Despite how much has been learned about Lyme borreliosis in the past decade, there are still many remaining areas of uncertainty. One fundamental question is whether persistent signs and symptoms of disease, despite the administration of what is currently considered to be adequate antibiotic therapy, are due to ongoing active borrelial infection, to a post-infectious syndrome, to irreversible sequelae of earlier tissue injury or due to a condition unrelated to Lyme disease. Reliable objective markers of infection, of clinical status and of host responses to the organism are required to discern the scope and the implications of persistent borrelial infection, the effectiveness of current treatment options, and the development of new therapeutic approaches. The goal of this study is to assemble and follow a well-characterized cohort of patients with post-treatment Lyme disease syndrome and relevant controls that will yield a prospective database upon which stringent diagnostic criteria can be established and future therapeutic trials can be designed.