Bronchiolitis obliterans organizing pneumonia (BOOP) is a ubiquitous entity, known to occur either idiopathically or in association with various pulmonary disorders. Histologically, it is characterized by myxomatous connective tissue plugs present in the lumen of bronchioles with extension into the alveoli. Its significance in lung allograft recipients is not well documented. The authors reviewed all post-lung transplant biopsies (565 transbronchial; 19 open), explanted lungs for retransplantation (six), and autopsies (38) from 115 patients. A total of 32 patients (18 females and 14 males) showed histological evidence of BOOP-like reactions (ie, Masson bodies in 44 transbronchial and seven open lung biopsies). The mean age was 47 years (range = 14 to 69 years). Sixteen patients were recipients of single lungs, 14 received bilateral single lungs, and two had heart and double-lung transplants. BOOP-like reactions (BOOP-LRs) occurred as early as day 5 and as late as day 1,208 (40 months) posttransplantation. Twenty patients had one biopsy showing BOOP-LR, of which three patients had resolving mild acute rejection, four had ongoing minimal acute rejection, seven had ongoing mild acute rejection, one each had ongoing moderate and bronchiolar rejection, and four showed associated cytomegalovirus (CMV) pneumonitis. Seven patients had two biopsies each of BOOP-LR of which six were associated with ongoing minimal or mild acute rejection, and one had resolving mild acute rejection. Three patients had three biopsies each of BOOP-LR all associated with ongoing minimal or mild acute rejection. Two patients had four biopsies each, showing BOOP-LR, with ongoing mild or moderate acute rejection or CMV pneumonitis. Forty of the total 115 lung transplant patients (34.8%) have developed bronchiolitis obliterans syndrome (BOS) or chronic airway rejection. Twelve of these patients are from the study group, of which five have a biopsy proven histological diagnosis of obliterative bronchiolitis (OB), and the remaining seven patients have been diagnosed clinically by deteriorating lung function tests. The authors conclude that BOOP-LR in the lung transplant setting result from acute epithelial injury secondary to either allograft rejection or an ongoing infection and are not a component of, nor do they necessarily predispose to, chronic rejection.