A 76-year-old man was admitted to the emergency room after being found disoriented by his family. He experienced episodes of fever (38-39.5  C) in previous days. There were no other complaints including headache or seizures. His prior history included an HIV-1 infection diagnosed 10 years before, pulmonary tuberculosis, hypertension and hyperthyroidism. His medications included zidovudine, lamivudine, efavirenz, tiamazol, and captopril. The last laboratorial results showed a CD4 cell count of 668 cells/mm and HIV viral load < 50 copies/mL. At admission, the physical examination showed a patient in good general conditions, moderately dehydrated and pale, afebrile, and hemodynamically stable. The neurological exam showed disorientation in time and space with a Glasgow Coma Scale of 14. The rest of the examination was normal. Laboratorial tests showed leukocytosis with left shift and no other alterations. A brain computed tomography scan showed a single low attenuation lesion with a uniform ring enhancement in the left temporal region (Fig. 1A). At this moment, empiric treatment for toxoplasmosis with parenteral trimethoprim-sulfamethoxazole (5/ 25 mg/ kg twice a day) was initiated. Seven days later, the patient's level of consciousness worsened, presenting disorientation and confusion. At this moment, after a review of the prior images, the hypothesis of bacterial or mycobacterial brain abscess was considered and ceftriaxone (2 g twice a day) and metronidazole (500 mg three times a day) were started. A lumbar puncture showed 1 cell/dL, 56 mg/dL of glucose, and 40 mg/dL of proteins. The cerebrospinal fluid direct exam and bacteria, fungus and mycobacteria cultures were all negative. The latex agglutination test for Cryptococcus neoformans,Streptococcus pneumoniae, Neisseria meningitides andHaemophylus influenzae type B, and the polymerase chain reaction (PCR) for these bacteria as well as for Mycobacterium tuberculosis were also all negative. Blood cultures performed by automated microbiology growth and detection system for aerobic, anaerobic and fungi resulted negative. The patient underwent trepanation, and drainage of the lesion was performed. Purulent material was obtained and sent to culture, which resulted in the growth of methicillin-sensitive S. aureus. Anti-Toxoplasma treatment was discontinued and antibiotic scheme was changed to oxacillin (2 g six times a day). The patient received eight weeks of this medication, improved progressively, clinically and radiologically (Fig. 1B-C) and was discharged to home. The patient underwent dental examination, with no evidence of dental infection. At transesophageal echocardiography the valves and left atrium were free of thrombi and vegetations, and there was no evidence of infection in thechest radiography and also in the abdominal ultrasound. In the daily assessment, measures of capillary glycemia were normal. Despite intensive efforts, no predisposing condition leading to the brain abscess, including trauma or surgery, intravenous drug use, skin or soft tissue infection, empyema, lung or intraabdominal abscess, bacterial endocarditis, dental infection, sinusitis or mastoiditis, were identified.