Patients entered in the study are randomly assigned to TMTX / LCV or to SMX/TMP for a 21-day trial. For the first 10 days, the trial is double-blind (neither patient nor physician knows which drugs the patient is receiving), and drugs are given by intravenous infusion. TMTX is given once every 24 hours and LCV every 6 hours; SMX/TMP is given every 6 hours. Doses are determined by body size. After the first 10 days, LCV and SMX/TMP may be given orally. Doses are adjusted or treatment is changed to intravenous PEN if side effects are too severe. During the 21-day trial, zidovudine (AZT) may not be used because of possible increased bone marrow toxicity. AZT may be resumed as soon as the patient's white cell count is acceptable. Aerosolized PEN therapy is begun 7 - 10 days after completion of therapy for the acute episode. PEN is inhaled once weekly for 4 weeks, then every 2 weeks for 48 weeks.