A 56-year-old man with a history of Wegener granulomatosis presented with 6 days of sinus congestion, fever, malaise, myalgias, episcleritis, and a morbilliform rash. An exacerbation of Wegener granulomatosis was the principal concern because of the frequency of flares in that disease. The patient developed acute renal failure, thrombocytopenia, transaminitis, and, finally, severe myocarditis that led to congestive heart failure. Additional history-taking and the evolution of his clinical features led to empirical treatment with doxycycline for human monocytic ehrlichiosis (HME). The diagnosis of HME was confirmed by both a polymerase chain reaction assay for Ehrlichia chaffeensis and by the demonstration of morulae within peripheral blood mononuclear cells. The patient improved promptly following institution of doxycycline, and his cardiac function returned to normal over a period of 4 months.