New treatments are needed to reduce the mortality rate from PCP in AIDS patients and to reduce the high relapse rate found after conventional therapy. Trimetrexate (TMTX) was chosen for this trial because it was found to be much more potent than sulfamethoxazole/trimethoprim (SMX/TMP) against the PCP organism in laboratory tests. Also TMTX, in combination with leucovorin (LCV), did not cause severe toxicity in a preliminary trial. It is believed that TMTX will be more effective than SMX/TMP in treating PCP and in preventing a recurrence of PCP. Preliminary studies suggest that aerosolized pentamidine (PEN) is likely to be effective in preventing a recurrence of PCP.