Three pediatric patients of a cohort of 24 who underwent allogeneic bone marrow transplantation (BMT) from matched unrelated or mismatched family member donors developed low grade fever and cough between 2 and 3 months after BMT in the absence of clinical GVHD. Imaging studies revealed bilateral patchy opacities and open lung biopsies revealed the characteristic histological appearance of bronchiolitis obliterans organizing pneumonia. All three patients were treated with steroids and in two patients the syndrome resolved over 1-2 months; the third patient developed progressive pulmonary failure and died 2 weeks after diagnosis. BOOP may be an under-recognized respiratory complication following BMT from a matched unrelated donor or mismatched family member.