We enjoyed Joshi and Miller's1 timely review article about doxycycline. However, there is a notable omission regarding the use of doxycycline that your readers should be reminded about. Doxycycline is generally considered the treatment of choice for the 2 recently described human ehrlichial infections: human monocytic ehrlichiosis, which is caused by Ehrlichia chaffeensis, and human granulocytic ehrlichiosis, which is caused by the as-yet-unnamed agent that closely resembles Ehrlichia equi and Ehrlichia phagocytophila.2,3 Both of these infections present as an acute nonspecific febrile illness.2,3 Human granulocytic ehrlichiosis appears to be transmitted by Ixodes ticks, which also transmit Borrelia burgdorferi, the pathogenetic agent of Lyme disease.4 Human granulocytic ehrlichiosis and human monocytic ehrlichiosis can be quite severe; particularly in the elderly. Fatalities have been reported.2,3 Response to doxycycline therapy generally occurs within l to 2 days.2,3,5 Approximately two thirds of patients with human granulocytic ehrlichiosis report having been bitten by a tick prior to the onset of the illness.3,5 Lyme disease and human granulocytic ehrlichiosis may occur simultaneously in areas where these infections are endemic.6 Doxycycline is the only antibiotic that can treat both infections. Indeed, doxycycline's activity against several emerging infections, including human monocytic ehrlichiosis, human granulocytic ehrlichiosis, Lyme disease, and, potentially, vancomycin-resistant enterococci, penicillin-resistant pneumococci, and mefloquine-resistant malaria, has led to its renaissance in the 1990s.