PAP is a rare alveolointerstitial lung disorder characterized histologically by the intra-alveolar accumulation of eosinophilic and PAS-positive material. We observed two cases of PAP after unrelated CB hematopoietic progenitor cell transplantation in children with ALL. No antagonist activity toward GM-CSF was identified in the patient tested. The putative multifactorial PAP etiology is discussed. This potentially curable condition should be considered in a CB allograft recipient with alveolointerstial lung disorder.