The drugs trimethoprim / sulfamethoxazole (TMP / SMX) and pentamidine, usually used to treat PCP in AIDS patients, have proven ineffective in many patients and have had to be discontinued in many other patients because of severe side effects. TMTX was chosen for this trial because it was found to be very active against the PCP organism in laboratory tests and, in a preliminary trial in combination with LCV, there was a high response rate without severe toxicity.