The pathological findings in the allografts of 14 children who underwent lung transplantation (LT) at St. Louis Children's Hospital, St. Louis, MO, in the period between July 1990 and May 1992 were reviewed. The study is based on histological analysis of 63 transbronchial biopsy (TBB) specimens, eight open lung biopsy specimens, and three pneumonectomy specimens. The mean age at transplantation was 10.5 years (range, 1 to 17 years) and the average follow-up period was 5.7 months. Sufficient tissue for an adequate pathological examination was obtained in 58 (92%) TBB specimens. Each specimen consisted of a mean of 6.12 tissue fragments, but only 4.79 fragments contained actual lung parenchyma for suitable examination. Ten patients (71%) had 23 biopsy-proven episodes of acute rejection with a frequency of 1.64 episodes per patient. The first episode was documented at a mean of 19 days after transplantation. Six patients (42.8%) developed bronchiolitis obliterans (BO). The definitive diagnosis of this condition was made either by open lung biopsy (n = 3) or on allograft pneumonectomy (n = 1), and it was infrequently recognized by TBB. Four of the six patients died less than 9 months after the diagnosis of BO was made, indicating the grave consequences of this complication. Two other deaths were attributed to the development of posttransplantation lymphoproliferative disorders.