Certain types and causes of pneumonia are unique to the immunocompromised host. The most frequent causes are cytomegalovirus, Pneumocystis carinii, varicella zoster virus, Candida species and Aspergillus species. Lymphoid interstitial pneumonia has recently been recognized in children with the acquired immunodeficiency syndrome. With the exception of varicella-zoster pneumonitis, an invasive procedure, such as open lung biopsy, is required to establish a definitive diagnosis. Infrequent causes of pneumonitis in immunocompromised children include Toxoplasma gondii; Cryptosporidium; Herpes simplex; adenovirus, gram-negative bacillary infections (Pseudomonas aeruginosa, Klebsiella pneumoniae, Escherichia coli, Legionella pneumophilia); Nocardia spp; zygomycetes, and Cryptococcus neoformans. The discovery of any of the aforementioned pneumonias suggests the patient may have a serious underlying immunodeficiency.