Symptomatic pulmonary toxoplasmosis is a relatively rare disease process, although the lung is frequently infected with the causative agent Toxoplasma gondii. Acute infection resulting in diffuse pneumonia is most likely in the immunosuppressed transplant recipient, and reinfection pneumonia occurs in this patient population as well as in patients with acquired immunodeficiency syndrome. The authors discuss the methods this protozoan uses to invade cells, to evade host defenses, and to cause tissue necrosis. The epidemiology of the disease relates directly to an inability to mount the effective cell-mediated immunity needed to keep the tissue cysts from undergoing effective replication into destructive tachyzoites. Alveolar macrophages and gamma interferon have been shown to be two important effector mechanisms in this control. Outcome is directly related to the ability to make an early diagnosis, which requires increased awareness of this disease in the patient populations at risk.