Figure. . Atypical erythema migrans lesion on a patient with PCR-positive result for Borrelia burgdorferi infection. The rash was not considered typical because it lacked central clearing and peripheral erythema. The differential diagnosis included a contact dermatitis and arthropod bite. At the initial examination, this patient was seronegative for B. burgdorferi by 2-tiered criteria. Three weeks after therapy, the patient had positive results for ELISA and IgM Western blot and negative results for IgG Western blot, providing evidence of seroevolution (i.e., increasing antibody titer and/or increase in band intensity or appearance of new antigen bands to B. burgdorferi).