To compare the safety of atovaquone (566C80) with intravenous (IV) pentamidine for the treatment of mild to moderate Pneumocystis carinii pneumonia (PCP) in AIDS patients who are intolerant of therapy with trimethoprim / sulfamethoxazole (TMP / SMX) by comparing the incidence of premature discontinuation of therapy due to toxicity. To compare the efficacy of 566C80 with intravenous (IV) pentamidine for the treatment of mild to moderate PCP in the same population.