A 2 year 9 month old boy was treated for a long time with a diagnosis of pneumonis. A biopsy of the neck mass performed under endotracheal anesthesia for 70 minutes. He was moved to the recovery room with an endotracheal tube, 50 min. later, cardiac arrest suddenly developed and was not resuscitated. Autopsy findings showed lymphosarcoma of the left lung with metastasis to the neck, liver and splsen, and a bilateral atypical pneunomia with left pleural malignant efficusion producing about 500ml. possible causes of death and the prevention are discussed.