A Phase II Open Randomized Comparison of 566C80 and Pentamidine Isethionate for the Treatment of Pneumocystis Carinii Pneumonia in AIDS Patients Who Are Intolerant of Trimethoprim / Sulfamethoxazole

Inclusion Criteria Patients must have the following: * History of, or are currently experiencing, intolerance to TMP / SMX, or to another sulfonamide, which required discontinuation of therapy. * Pneumocystis carinii pneumonia (PCP). * Willing and able to give informed consent. Exclusion Criteria Co-existing Condition: Patients with the following conditions or symptoms are excluded. * Malabsorption disorder or vomiting that would, in the judgement of the 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. * Present occurrences of abnormal prolongation of QT interval on standard, 12 lead EKG. Concurrent Medication: Excluded: * Drugs with potential anti-pneumocystis effect (eg: * sulfonamides, dapsone, trimethoprim, other dihydrofolate reductase (DHFR) inhibitors, primaquine, clindamycin, sulfonylureas). * Ganciclovir. * Zidovudine. * Investigational agents including anti-retroviral agents (ddI, ddC, etc.). Patients receiving these drugs prior to entry must discontinue their use during the therapy phase (21 days) of the trial. * Corticosteroids (except replacement therapy) during the 21 day treatment period (Strata A and C Patients). * Class 1A antiarrhythmics (ie: * quinidine, procainamide, disopyramide). Patients with the following are excluded: * Judged by the investigator to be in impending respiratory failure. * Known intolerance to parenteral pentamidine isethionate, or therapeutic failure with same for treatment of current episode of PCP. * Significant psychosis or emotional disorder that, in the investigator's opinion, would preclude the patient from adhering to the protocol. * Inability or unwillingness to take medication orally or with food. * Prior documented glucose-6-phosphate dehydrogenase (G6PD) deficiency. * History, or present occurrences, of abnormal prolongation of QT interval on standard, 12 lead EKG. * Termination from FDA 053A due to toxicity. * For patients entering under historic intolerance criteria (Groups A and B), prior therapy for this episode of PCP is an exclusion. Prior Medication: Excluded: * Treatment within 4 weeks of entry for a prior episode of PCP. * For patients entering under historic intolerance criteria (Groups A and B), prior therapy for this episode of PCP is an exclusion. Required: * Adjuvant prednisone for patient enrolled in Strata B or D.