Lyme disease, caused by infection with Borrelia burgdorferi, can affect those exposed to a vector tick. Pregnant women are no exception, and such infection places the fetus at risk. It is particularly important to recognize the disease early so that effective therapy may be instituted. Although the present patient had a favorable outcome, not all do. Clinical diagnosis is especially important since conventional laboratory tests may be inadequate or require lengthy periods of time before a positive result occurs. The dermatologic sign of Lyme disease, erythema migrans, although occurring in only 50 percent of cases, is likely to be the most important diagnostic sign.