We reviewed all tissue specimens from 163 transplant patients (108 double lung transplant [DLT], 55 single lung transplant [SLT]) between November 1983 and January 1994 for abnormalities indicating bronchiolitis obliterans organizing pneumonia (BOOP) and found 17 cases (14 DLT and 3 SLT). Of the three SLTs, BOOP was diagnosed by open lung biopsy (OLB) in two and one was found at autopsy. Of the 14 DLTs, BOOP was diagnosed by transbronchial biopsy (TBB) specimens (9), OLB specimens (2), autopsy (1), TBB and OLB specimens (1), and OLB specimens and autopsy (1). BOOP was found between 1 and 43 months posttransplantation; time of survival from diagnosis was between 2 and 36 months with 9 patients presently alive. Concurrent pathologic diagnosis at the time of BOOP findings were as follows: acute rejection (7) (grade 1 [4] and grade 2 [3]), BO and grade 1 rejection (2), BO and grade 2 rejection (2), BO and Aspergillus infection (1), acute alveolar injury (1), acute alveolar injury and pulmonary embolus (1), acute rejection (grade 1) and Burkholderia cepacia pneumonia (1). No other pathologic diagnosis was found in 1 patient. In total, 11 of 17 patients (65%) had associated acute rejection. Of the 17 patients, 7 subsequently developed BO and 3 had BO before the finding of BOOP. Death occurred in 8 patients (5 DLT and the 3 SLT) between 2 and 6 months after the diagnosis. We conclude that BOOP is an important complication after lung transplantation; it was present in 13% of DLTs and 5% of SLTs. BOOP was most often associated with acute rejection.