BACKGROUND Bronchiolitis obliterans syndrome (BOS) is the main cause of late morbidity and mortality in lung transplantation. This study was designed to accurately determine the prevalence of this syndrome of chronic lung allograft dysfunction (which is presumed to be due to chronic rejection). METHODS A retrospective analysis was done of 212 consecutive lung transplantations performed at Barnes Hospital between July 1988 and March 1994 to characterize the prevalence and course of BOS. One hundred eighty-seven transplant recipients survived at least 3 months after transplantation, putting them at risk for BOS. Recipients free of BOS (group I) were distinguished from those with BOS (group II) based on the presence of declining spirometry (forced expiratory volume in 1 second persistently less than 80% of previous baseline) or histologic obliterative bronchiolitis in group II. RESULTS There were 110 transplantations in group I (59%) and 77 in group II (41%). At follow-up, BOS was detected using the following criteria: declining forced expiratory volume in 1 second alone, 40 of 77 (52%); positive histologic results alone, 7 of 77 (9.1%); and both, 30 of 77 (38.9%). Declining spirometry was the most common initial sign of BOS onset (57 of 77, 74%). There were no differences between groups with respect to age, sex, indication for transplantation, or type of transplantation performed. The mortality rate was significantly higher with BOS (group II, 22 of 77 [28.6%] versus group I, 8 of 110 [7.3%]; p = 0.001) and was not related to either the type of transplantation performed or the indication for transplantation. Follow-up of group II (mean 35.1 months; range, 7.1 to 63.7 months) showed a delay until BOS onset (16.1 +/- 1.2 months); when BOS was fatal, death ensued within 11.5 +/- 2.4 months of its onset. Comparison of the first and last quartiles of recipients in this series (QTR1 versus QTR4, 53 patients in each) demonstrated a higher prevalence of BOS in QTR1 (24 with BOS of 43 at risk [55.8%] versus QTR4, 5 with BOS of 52 at risk [9.6%]; p < 0.001) and a worse BOS functional score in QTR1 (2.2 +/- 0.2 versus QTR4, 0.8 +/- 0.2; p = 0.007). CONCLUSIONS (1) Bronchiolitis obliterans syndrome is truly a clinical syndrome, not simply a pathologic entity; (2) BOS displays considerable latency in onset and progression; (3) lung transplant recipients must therefore be followed up for a sufficient interval to determine the actual prevalence and mortality rate of BOS; and (4) the prevalence and mortality rates of BOS are higher than previously appreciated, exceeding 50% and 40%, respectively.