Original design: The drugs usually used to treat PCP in AIDS patients, trimethoprim / sulfamethoxazole and pentamidine, have had to be discontinued in many patients because of severe side effects. Currently there are no proven alternatives to these drugs. TMTX was chosen for this trial because it was found to be very active against the PCP organism in laboratory tests. Also TMTX, in combination with LCV, had a high response rate and did not cause severe toxicity in a preliminary trial.