Lyme disease, caused by a tick-transmitted spirochete, typically begins with a unique skin lesion, erythema chronicum migrans. Of 314 patients with this skin lesion, almost half developed multiple annular secondary lesions; some patients had evanescent red blotches or circles, malar or urticarial rash, conjunctivitis, periorbital edema, or diffuse erythema. Skin manifestations were often accompanied by malaise and fatigue, headache, fever and chills, generalized achiness, and regional lymphadenopathy. In addition, patients sometimes had evidence of meningeal irritation, mild encephalopathy, migratory musculoskeletal pain, hepatitis, generalized lymphadenopathy and splenomegaly, sore throat, nonproductive cough, or testicular swelling. These signs and symptoms were typically intermittent and changing during a period of several weeks. The commonest nonspecific laboratory abnormalities were a high sedimentation rate, an elevated serum IgM level, or an increased aspartate transaminase level. Early Lyme disease can be diagnosed by its dermatologic manifestations, rapidly changing system involvement, and if necessary, by serologic testing.