In 1989, the United States Public Health Service convened a Task Force of experts to consider the expanding knowledge base about prevention of Pneumocystis carinii pneumonia (PCP) among adults and adolescents (greater than or equal to 13 years of age) with human immunodeficiency virus (HIV) infection. This Task Force concluded that the morbidity, mortality, and cost due to PCP could be substantially reduced by appropriate use of antipneumocystis prophylaxis in subgroups of HIV-infected patients known to be at high risk, and developed recommendations for the administration of prophylactic regimens (1). The recommendations state that CD4+ T-lymphocyte counts should be monitored prospectively at 3- to 6-month intervals and prophylaxis should be instituted when patients become immunologically susceptible to PCP. Susceptibility was defined by a CD4+ T-lymphocyte count less than 200 cells/microliters or less than 20% of total circulating lymphocytes, or the occurrence of a previous episode of PCP. The goal of this approach was to reduce the frequency both of initial episodes of PCP (primary prophylaxis) and of relapses or recurrences (secondary prophylaxis). Either oral trimethoprim-sulfamethoxazole (TMP-SMX) or aerosol pentamidine was recommended for prophylaxis, but because direct comparative data were lacking, neither regimen was endorsed as "preferred." Since the recommendations were issued in 1989, additional information has become available about the efficacy and safety of aerosol pentamidine and oral TMP-SMX. A trial sponsored by the National Institute of Allergy and Infectious Diseases AIDS Clinical Trials Group compared these two regimens in a prospective randomized study; in August 1991, this study was terminated by an independent data and safety monitoring board because statistically significantly fewer recurrences of PCP were observed in the oral TMP-SMX group than in the aerosol pentamidine group (2). On the basis of this finding and other studies assessing PCP prophylaxis, the Task Force was reconvened on October 5, 1991. This report contains the revised recommendations issued by the Task Force.