Patients are randomized into one of two treatment groups to receive either (1) 566C80 for 21 days, or (2) SMX/TMP for 21 days. Patients will be stratified according to severity of PCP. Group A will be those with an arterial-alveolar (A-a) DO2 < 35 mm Hg. Group B will have an A-a DO2 of 35-45 mm Hg., and will also be required to receive therapy with Corticosteroids. All doses are taken with food. During the 21 days of treatment, patients are examined clinically for adverse effects and have hematology (blood-related) and clinical chemistry studies conducted a minimum of 2 times weekly. More frequent monitoring may be required at the discretion of the investigator. To evaluate the effectiveness of study medication, the clinical status of each patient is evaluated 2 to 3 times per week (e.g., dyspnea score, cough score, chest tightness/pain score, vital signs). Also, on days 7 and 21 of treatment, an arterial blood gas measurement and chest X-ray are performed. Patients who experience severe toxicities will be discontinued from the study and placed on alternative therapy. Patients will also be removed from study if they show significant clinical deterioration within the first 7 days of therapy or if there is no improvement after 10 days of therapy. This study involves a double placebo with one group randomized to receive oral 566C80 and placebo tablets which look like SMX/TMP while the other group will receive SMX/TMP and placebo tablets looking like 566C80.