Pneumocystis carinii pneumonia (PCP) is an opportunistic infection associated with increased morbidity and mortality in solid-organ and bone-marrow transplant recipients. Side effects of trimethoprim-sulfamethoxazole (TMP/SMX) are frequent; therefore, we performed a preliminary study using atovaquone suspension, 750 mg once daily, for 1 year for the prevention of PCP in liver transplant recipients intolerant to TMP/SMX therapy. Twenty-eight patients were treated, and data were analyzed for efficacy and toxicity. Adverse events occurred in 14 subjects, mainly related to the gastrointestinal tract. Side effects from TMP/SMX, i.e., rash, completely resolved and bone-marrow suppression improved in 62% of patients. No patients developed Pneumocystis carinii infection. Although a lower dose of atovaquone once daily may be effective in transplant recipients, further studies are necessary to confirm this preliminary observation.