A 74-year-old woman visited an otolaryngology clinic with pharyngeal pain, and was diagnosed with a peritonsillar abscess. She received antibiotics and underwent incisional drainage, but displayed high white blood cell and blast cell counts, and was referred to our hospital. Gram-negative rods (Leptotrichia trevisanii) were detected in blood cultures performed on admission. She was diagnosed with bacteremia and acute myelogenous leukemia (FAB classification: M1). After antibiotic therapy, she temporarily recovered from the bacteremia, but subsequently died on day 34. Although Leptotrichia trevisanii bacteremia is extremely rare, clinicians should consider it in cases involving immunocompromised patients with oral lesions.