We conclude that Lyme disease is not a common cause of idiopathic heart failure in the Midwestern United States and that false-positive Lyme disease serologic results are not rare among patients with severe heart failure. Patients with significant cardiac disease who are found to be EIA seropositive should have confirmatory Western blots performed before consideration of treatment. Based on our findings, we cannot recommend either the routine serologic screening of patients with idiopathic cardiomyopathy or aggressive (e.g., parenteral) antibiotic treatment of seropositive patients unless the specific clinical history suggests antecedent Lyme disease.