Gradual Initiation of Sulfamethoxazole/Trimethoprim as Primary Pneumocystis Carinii Pneumonia Prophylaxis

Inclusion Criteria Concurrent Medication: Allowed if clinically indicated: * Recombinant erythropoietin (rEPO) and G-CSF. Allowed for symptomatic treatment of mild study drug toxicity: * Antipyretics and analgesics (ibuprofen). * Antihistamines (diphenhydramine HCl). * Terfenadine or astemizole (but not allowed with concomitant antifungal or macrolide use). * Systemic steroids. Patients must have: * HIV infection. * CD4 count \<= 250 cells/mm3 OR history or presence of thrush. * No history of confirmed or probable pneumocystosis. NOTE: * Pregnant women are not excluded, but safety issues should be discussed with patient prior to enrollment. * This study is appropriate for prisoner participation. * Coenrollment in ongoing ACTG antiretroviral studies is permitted provided no new study drugs are added to the patient's drug regimen for 4 weeks before or after initiation of SMX/TMP. Prior Medication: Allowed: * Prior aerosolized pentamidine and dapsone for primary PCP prophylaxis. Exclusion Criteria Co-existing Condition: Patients with the following symptoms or conditions are excluded: * Known adverse reactions to sulfa, trimethoprim, or SMX/TMP. * Inability to comply with dosing schedule or complete dosing record. Concurrent Medication: Excluded: * Procysteine. * Glutathione. * N-acetylcysteine (NAC). * Antihistamines (unless used for symptomatic treatment of study drug toxicity). * Systemic corticosteroids (unless used for replacement purposes). * Leucovorin calcium (unless used for symptomatic treatment of study drug toxicity). * TMP or sulfa drugs outside of the study. Prior Medication: Excluded at any time: * Prior SMX/TMP as primary PCP prophylaxis. Excluded within 4 weeks prior to study entry: * Initiation of antiretroviral agents. * Initiation of anti-infective agents (including SMX/TMP for another indication). Excluded within 2 weeks prior to study entry: * Antihistamines. * Procysteine. * Glutathione. * N-acetylcysteine (NAC). * Systemic corticosteroids (unless used for replacement purposes). * Leucovorin calcium. * TMP and sulfa drugs separately.