Background: Lyme disease has a wide spectrum of clinical manifestations. Diagnosis is usually based on the clinical and serologic picture rather than on microbiological confirmation.

Objective: To examine the clinical presentation and treatment outcome of early Lyme disease in patients with microbiologically confirmed erythema migrans.

Design: Observational cohort study.

Setting: 31 university-based or clinician-practice sites in 10 endemic states.

Participants: 10 936 participants enrolled in a phase III trial of Lyme disease vaccine; 118 participants had erythema migrans in which Borrelia burgdorferi was detected by culture or polymerase chain reaction.

Measurements: Clinical characteristics and treatment outcome were noted. Skin biopsies of erythema migrans were performed for culture and detection of B. burgdorferi by polymerase chain reaction; serologic responses were determined by Western blot.

Results: The 118 patients with microbiologically confirmed erythema migrans presented a median of 3 days after symptom onset. Early erythema migrans commonly had homogeneous or central redness rather than a peripheral erythema with partial central clearing. The most common associated symptoms were low-grade fever, headache, neck stiffness, arthralgia, myalgia, or fatigue. By convalescence, 65% of patients had positive IgM or IgG antibody responses to B. burgdorferi. Most patients responded promptly to antibiotic treatment.

Conclusions: In major endemic areas in the United States, Lyme disease commonly presents as erythema migrans with homogeneous or central redness and nonspecific flu-like symptoms. Clinical outcome is excellent if antibiotic therapy is administered soon after symptom onset.