Inclusion Criteria

Patient must have the following:

• Presumptive diagnosis of AIDS as defined by the CDC.
• Untreated Pneumocystis carinii pneumonia (PCP).
• Willingness and ability to give informed consent.

Prior Medication:

Allowed:

• Prophylactic therapy for Pneumocystis carinii pneumonia (PCP) including aerosolized pentamidine or sulfamethoxazole/trimethoprim (SMX/TMP) (at a dose no greater than two DS tablets twice daily).

Exclusion Criteria

Co-existing Condition:

Patients with the following conditions or symptoms are excluded:

• Judged by the investigator to be in impending respiratory failure.
• Malabsorption or vomiting that would, in the judgment of investigator, potentially limit the retention and absorption of an oral therapy.
• Concurrent bacterial, fungal, or viral pneumonitis, pulmonary Kaposi's sarcoma or other concurrent illness, or chronic pulmonary disease that, in the investigator's opinion, would make interpretation of drug efficacy difficult.

Concurrent Medication:

Excluded:

• Corticosteroid treatment (except replacement therapy or patients in Group B).
• Ganciclovir.
• Zidovudine (AZT).
• Investigational agents including antiretroviral agents (didanosine (ddI), dideoxycytidine (ddC), etc.).

Drugs likely to have anti-pneumocystis effect such as:

• Sulfonamides.
• Pentamidine.
• Dapsone.
• Trimethoprim.
• Other DHFR inhibitors.
• Primaquine.
• Clindamycin.
• Sulfonylureas.

Patients with the following are excluded:

• Judged by the investigator to be in impending respiratory failure.
• Prior therapy for this episode of PCP or treatment within 4 weeks of entry for a prior episode of PCP.
• Unable to or refuse to discontinue zidovudine, ganciclovir, or other antiretroviral agents during the 21 day treatment period.
• Unable to take medication orally or unwilling or unable to take study medication with food.
• Significant psychosis or emotional disorder such that, in the investigator's opinion, the patient would not be compliant with the study protocol.
• Prior documented glucose-6-phosphate dehydrogenase (G6PD) deficiency.
• Prior history of life-threatening toxicity to SMX/TMP such as severe rash or Stevens-Johnson syndrome.

Prior Medication:

Excluded:

• Prior therapy for this episode of Pneumocystis carinii pneumonia (PCP) or treatment within 4 weeks for a prior episode of PCP.
• Blood transfusions.