The primary objective of the study is to evaluate the effectiveness of l-leucovorin in preventing toxicity from high dose trimethoprim / sulfamethoxazole (TMP / SMX) used as a therapy for Pneumocystis carinii pneumonia (PCP) in patients with AIDS.

Inclusion Criteria

Patients must have the following:

• Diagnosis of PCP.
• Fit the CDC definition of AIDS.
• Be receiving intravenous or oral trimethoprim / sulfamethoxazole (Bactrim, Septra) in doses of = or > 15mg/kg/day of the trimethoprim component.
• Be receiving = or < 48 hours of trimethoprim / sulfamethoxazole (TMP / SMX) prior to randomization.
• Must sign informed consent in accordance with FDA guidelines.

Exclusion Criteria

Co-existing Condition:

Patients with the following conditions or symptoms are excluded:

• Cannot take oral medications.
• Require continued administration of anticonvulsant agents such as phenytoin, phenobarbital or primidone.

Concurrent Medication:

Excluded:

• Continued administration of anticonvulsant agents such as phenytoin, phenobarbital or primidone.

Patients with the following are excluded:

• Cannot take oral medications.
• Require continued administration of anticonvulsant agents such as phenytoin, phenobarbital or primidone.

Prior Medication:

Excluded:

• > 48 hours of trimethoprim / sulfamethoxazole prior to randomization.