Acute respiratory failure (ARF) in an 11-year-old child with pre-T acute lymphoblastic leukemia (ALL) at the beginning of induction therapy was observed, connected with a pulmonary thrombosis and not with an infective origin. A systematic search for this pathology identified six other children with the same pulmonary complication, five of whom where in the early phase of acute nonlymphoblastic leukemia (ANLL) and one in induction therapy for ALL in marrow relapse. At the beginning of the symptomatology, all children presented severe hypoxia and hypercapnia, with no or minimal chest radiograph abnormalities and no clear hemodynamic involvement. In all patients the arteriography and nuclear imaging studies confirmed the diagnosis. The causes of the thrombi could be connected with neoplastic emboli after cell lysis and/or with the vascular damage resulting from antiblastic therapy. Intravenous urokinase treatment and respiratory assistance had been successfully carried out in six of seven children.