To establish experimental Pneumocystis carinii infection in simian immunodeficiency virus (SIV)-infected macaques as a model of acquired immunodeficiency syndrome (AIDS)-associated P. carinii pneumonia (PCP), SIV-infected macaques were inoculated intrabronchially with macaque-derived P. carinii, and P. carinii-specific polymerase chain reaction (PCR) and flow cytometric analysis of bronchoalveolar lavage fluid were done biweekly for up to 44 weeks after inoculation. All inoculated animals had a P. carinii-specific PCR product after infection. CD8(+) T cells in lung lavage samples from SIV- and P. carinii-coinfected animals increased to >90% of total CD3(+) cells, a pattern associated with naturally acquired P. carinii infection. Progression of disease also was correlated with increased neutrophil infiltration to the lungs. The animals had a protracted period of asymptomatic colonization with P. carinii before progression to PCP. The development of a model of PCP in SIV-infected rhesus macaques provides the means to study AIDS-associated PCP.