Following the initial observation by Dr. Margaret Fischl that trimethoprim-sulfamethoxazole can prevent Pneumocystis carinii infection in patients with Kaposi's sarcoma, initiating prophylaxis for pneumocystic infection in all patients with less than 200 CD4+ cells/mm3 has become accepted practice. This prophylactic intervention has been found not only to reduce the development of pneumonia due to P. carinii but also to prolong life. Drs. Henry Masur and Joseph A. Kovacs first reviewed prophylaxis for P. carinii pneumonia in patients infected with the human immunodeficiency virus for the AIDS Commentary 3 years ago. They have updated that initial review for this AIDS Commentary, placing currently available information into concise clinical perspective and detailing a rational plan for the clinician to follow based on results of recent studies.