Anaerobic infections are quite rare in pediatric age, being that, they affect only neonates and immunodepressed patients. We think to be somewhat interesting to describe the case of our patient, a 9 year old boy, unaffected by any predisposing factor, came under our observation because of a severe respiratory distress. He showed evident clinical and radiological signs of pleural effusion in the right lung, together with a gas coil in the upper field and a left mediastinal shifting. A thoracentesis was then performed, giving rise to 600 ml of foul smelling purulent material; this procedure promptly improved his respiratory function. A permanent drainage trough the chest wall was set and an antibiotic therapy, based on the clinical picture and the character of the exudate, begun. In effect, the typical smell of the purulent material led us to suspect an anaerobic infection, and for this reason we employed the teicoplanin iv, a rarely used in the pediatric age drug. While blood cultures were negative for any organism, exudate cultures yielded Peptostreptococcus anaerobius; the last one resulted highly sensible following antibiogram to the previously chosen drug. The x-ray pattern and the rapid disappearing of the gas coil induced us to exclude further either congenital or acquired lung diseases. We conclude that, in absence of other proved sources of entry, the air presence in the pleural space was secondary to gas formation by the anaerobic micro-organism. The clinical course was very satisfactory allowing the patient to be dismissed on the 28th hospital day, with no need of further surgical therapy.(ABSTRACT TRUNCATED AT 250 WORDS)