Pulmonary nocardiosis is an uncommon but serious infection that is increasingly found in immunosuppressed persons, especially transplant recipients and persons with AIDS. The Nocardia species are denizens of soil and decaying plants that gain entry to humans through inhalation or inoculation. Pulmonary nocardiosis typically presents as an acute to subacute necrotizing pneumonia, with a variable clinical picture. Metastatic infections of the brain and subcutaneous tissues are common complications. Most clinical laboratories can isolate these microorganisms, but final speciation may be a challenge and antimicrobial susceptibility testing is especially difficult because of the slow rate of growth of Nocardia species. Full identification of species and susceptibility testing is important because of the epidemiologic implications and the difficulties of successfully treating these infections in immunosuppressed patients. Sulfonamides, including trimethoprim-sulfamethoxazole, remain the most reliable antimicrobials. Many alternative agents are active against Nocardia in vitro, but clinical data are limited.