Prophylaxis against Pneumocystis carinii pneumonia is a logical management strategy for patient populations that have a high incidence of this infectious complication. Appropriate populations for consideration would include those with human immunodeficiency virus infection, recent recipients of bone marrow or solid organ transplants, children with acute lymphocytic leukemia, and adults with T cell leukemias. Trimethoprim-sulfamethoxazole given twice daily (2.5 mg of trimethoprim/kg and 12.5 mg of sulfamethoxazole/kg every 12 hours on three or seven consecutive days per week has been shown to be effective and well tolerated in some patient populations. Pyrimethamine-sulfadoxine given once weekly and aerosolized pentamidine given once or twice monthly are promising prophylactic agents that are currently undergoing closer scrutiny to determine their efficacy and safety.