We review Pneumocystis carinii pneumonia (PCP) in patients in the developing world (i.e., Africa, Asia, the Philippines, and Central and South America) who have acquired immunodeficiency disease (AIDS). During the first decade of the AIDS pandemic, PCP rarely occurred in African adults. More recent reports have noted that PCP comprises a significantly greater percentage of cases of pneumonia than it did in the past. This trend dramatically contrasts with that observed in industrialized nations, where a reduction in the number of cases of PCP has occurred as a result of the widespread use of primary P. carinii prophylaxis and highly active antiretroviral therapy. Throughout the developing world, the rate of coinfection with Mycobacterium tuberculosis and PCP is high, ranging from 25% to 80%. Initiation of treatment when PCP is in an advanced stage may account for the high mortality rates (20%-80%) associated with pediatric PCP in the developing world.