Trimethoprim-sulfamethoxazole (TMP-SMZ) is widely prescribed as prophylaxis for Pneumocystis carinii pneumonia (PCP) in human immunodeficiency virus (HIV)-infected persons. Its efficacy against other infections has not been thoroughly evaluated. To compare the risk for infectious diseases for persons who were prescribed TMP-SMZ with that for patients who were not prescribed TMP-SMZ, we examined data collected from the medical records of HIV-infected patients (January 1990 through September 1999) who were enrolled in the Adult and Adolescent Spectrum of HIV Disease Project. During intervals when patients had CD4(+) T lymphocyte counts of <200 cells/microL (19,081 persons; 22,801 person-years), prescription of TMP-SMZ was associated with significant protection from toxoplasmosis, salmonellosis, infection with Haemophilus species, invasive or any staphylococcal infection, and PCP, but not from Shigella, pneumococcal or nonpneumococcal Streptococcus, Klebsiella, or Pseudomonas species. We demonstrate that prescription of TMP-SMZ for PCP prophylaxis in persons with HIV infection is associated with significantly decreased risk for several infectious diseases. These findings may be of interest to HIV prevention programs in resource-poor countries.