Legionella micdadei has previously been described as a cause of nosocomial pneumonia, particularly in kidney transplant recipients. Cell-mediated immunity is the principal host defense against this pathogen. A common clinical scenario in the immunocompromised host is that of septic pulmonary embolus, but asymptomatic infections have also been reported. The organism is weakly acid-fast in clinical specimens but loses this property when grown on solid media. We report two cases of L. micdadei pneumonia, differing markedly in clinical severity and outcome, in bone marrow transplant recipients. Additionally, we note the growth of the organism in liquid culture media with preservation of its acid-fast property. © 1990 by the The University of Chicago. All rights reserved.