BACKGROUND Aerococcus viridans are Gram-positive, catalase and oxidase-negative, microaerophilic, and non-motile bacteria species that are rarely associated with human infections such as arthritis, bacteremia, endocarditis, and meningitis. The bacteria are also fastidious (i.e., have complex nutritional requirements) and often confused with Streptococci species or treated as a contaminant. CASE REPORT We report a case of Aerococcus septicemia in an 85-year-old female patient, who transferred from a nursing home to an acute care hospital in Washington DC, USA. She had a 2-day history of worsening mental status, fever of 38.9°C (102°F), and tachycardia. Urinalysis revealed numerous white blood cells and bacteria. Laboratory tests revealed a white blood cell count of 14 000 cells/mL (85% neutrophils, 8% lymphocytes, 5% bands, and 2% monocytes), hemoglobin of 12.6 g/dL, and serum creatinine of 0.8 mg/dL. Blood and urine cultures obtained during admission grew penicillin-resistant A. viridans, identified via matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF) on day 3 of admission. The patient received empiric vancomycin with piperacillin-tazobactam, and we deescalated to vancomycin monotherapy to complete a 14-day treatment course. CONCLUSIONS This case report highlights the role of MALDI-TOF for identifying fastidious organisms, and we were able to form a better clinical correlation between patient symptoms and causative organisms. We believe that antimicrobial therapy (in accordance with susceptibility results) should be initiated in symptomatic patients who have A. viridans isolated in significant amounts in urine or from a sterile site.