An 11-year-old boy is brought to the Emergency Department with a history of a dental abscess in the upper jaw. He was previously treated with Amoxicillin, Clavulanate plus Clindamycin, without any improvement. He complained of limb weakness, paresthesias, abnormal gait and urinary hesitancy. On physical examination he had swelling of the face on the right side, with slight homolateral proptosis. The neurologic examination revealed decreased muscular strength in the limbs; deep tendon reflexes were brisk and symmetric throughout and sensory function was apparently normal. The laboratory analysis showed leukocytosis, neutrophylia, elevated erythrocyte sedimentation rate and lactate dehydrogenase. The patient was hospitalized, keeping the antibiotics previously prescribed. On day 3 he started lowgrade fever with no improvement of the clinical situation. A maxillofacial CT was performed, showing opacification of the sinuses, presence of an expansive lesion occupying the right maxillary sinus with erosion of the wall, extending to the orbit, infratemporal and buccal areas. He underwent an Endoscopic Sinusectomy, with washing of the right maxillary sinus, and right anterior ethmoidectomy with drainage of hemopurulent liquid. Clindamycin was suspended and he was started on Metronidazole, Netilmicin and Prednisolone. Although apyretic, he kept complaints of pain and limb weakness, accompanied by nocturnal sweating. A Cervical-thoracic MRI was performed, showing a paravertebral mass, spinal canal infiltration, alteration of bone trabeculation and insuflation of the vertebral body at D5 level. The diagnosis of FAB L3 Acute Leukemia was established based on more than 30% blasts in the bone marrow aspirate. The patient underwent chemotherapy.