Pneumocystis carinii pneumonia (PCP) is the most common opportunistic infection in children who have acquired immunodeficiency syndrome (AIDS). Despite the publication of guidelines for prophylaxis against PCP for children infected with human immunodeficiency virus (HIV) in 1991 (1), ongoing AIDS surveillance has detected no substantial decrease in PCP incidence among HIV-infected infants. Studies indicate that this continued incidence is associated with failure to identify HIV-infected children before PCP occurs and with limitations in the ability of CD4+ measurements to identify children at risk for PCP. In March 1994, the National Pediatric & Family HIV Resource Center, in collaboration with CDC, convened a working group to review additional data about the occurrence of PCP among HIV-infected children and to reevaluate the 1991 PCP prophylaxis guidelines for children. This report summarizes these new data and presents revised PCP prevention guidelines that recommend a) promptly identifying children born to HIV-infected women and initiating regular diagnostic and immunologic monitoring of such children; b) beginning PCP prophylaxis at 4-6 weeks of age for all children who have been perinatally exposed to HIV; c) continuing prophylaxis through 12 months of age for HIV-infected children; and d) making decisions regarding prophylaxis for HIV-infected children > or = 12 months of age based on CD4+ measurements and whether PCP previously has occurred.