Inclusion Criteria

Patients must have:

• HIV infection.
• CD4 count <= 200 cells/mm3 OR a history of prior PCP.
• No active pneumocystosis. Patients or their guardians must sign informed consent. Pregnant patients are eligible at the clinician's discretion. Patients who do not meet required laboratory values may be eligible at the discretion of the clinician.

Exclusion Criteria

Co-existing Condition:

Patients with the following symptoms and conditions are excluded:

• Known treatment-limiting reaction to sulfonamides or trimethoprim.

Concurrent Medication:

Excluded:

• Other PCP prophylaxis or medication with anti-PCP activity.