Toxoplasmosis is the most common opportunistic infection of the central nervous system in patients with AIDS. The standard treatment for toxoplasmic encephalitis is pyrimethamine and sulfadiazine. There have been few reports of concurrent Toxoplasma brain abscess and cavitary Pneumocystis carinii pneumonia (PCP) in Taiwan. We report the case of a 26-year-old homosexual man with coexisting infection with Toxoplasma gondii and P. carinii who was successfully treated for brain abscess with clindamycin and sulfadiazine. The cavitary lung lesions, initially diagnosed as pulmonary tuberculosis, were proved to be PCP by lung biopsy. HIV infection and syphilis had been diagnosed 1 year before admission. He presented with general weakness, ataxia, nausea, blurred vision and fever for 2 weeks. Magnetic resonance imaging of the brain revealed multiple ring-enhanced lesions over the cerebrum and cerebellum. Chest roentgenography showed a 3-cm lesion with cavitation over the right upper lung field. Diagnostic computerized tomography-guided lung biopsy revealed P. carinii cysts. Clindamycin, sulfadiazine and trimethoprim (TMP)-sulfamethoxazole (20 mg/kg/day TMP) were given with good response. His CD4 count rose from 40 to 280/microL 4 months later. All antibiotics were discontinued after 4.5 months due to the development of a skin rash. He was well at follow-up 1 year later. This case suggests that the combination of clindamycin and sulfadiazine is an effective treatment for Toxoplasma brain abscess and highlights the importance of diagnostic lung biopsy for cavitary lung lesions, particularly in a region endemic for tuberculosis.