Pulmonary complications (PC) remain a significant barrier to the success of allogeneic blood and marrow transplantation (BMT). Pretransplant pulmonary function tests (PFTs) have been correlated with risk of early respiratory failure and mortality in adult BMT recipients. There is limited data on their relationship to posttransplant outcomes in pediatric patients. We sought, in pediatric allo-BMT recipients (1) to analyze the spectrum of infectious and noninfectious PCs, (2) to evaluate the prevalence and course of PFT abnormalities before and after transplant and (3) to correlate pretransplant PFT findings with patient outcomes, specifically risk of PC, respiratory failure, and death. We conducted a retrospective review of PC in all patients who underwent allo-BMT at Children's Hospital of Pittsburgh during 1996 to 2006. PFTs were performed in children 6 years and older pretransplant, 3, 6, 12, and 24 months after transplant. PCs occurring within 100 days of BMT were considered early. One hundred ten consecutive children who underwent allo-BMT were included (median age = 9.7 years; 67 males, 43 females). Seventy-five of I 10 patients had 370 PFT studies performed; 62 of 73 patients >6 years of age (85%) underwent PFT studies pre-BMT There was a significantly higher risk of early respiratory failure in patients with reduced pretransplant forced expiratory volume in 1 second (FEV1) (P = .0001, odds ratio [OR] 5.1) or forced vital capacity (FVC) (P = .0001, OR 8.5). Forty-three of 110 (39%) patients required mechanical ventilation, and in 30 episodes (70%), patients remained ventilator-dependent until time of death. Posttransplant, we observed statistically significant reductions in FEV1, FVC, total lung capacity (TLC), and diffusing capacity of the lungs (DLco) at 3 months post-BMT and similar reductions at 6 months post-BMT except for DLco (not significant). Between 12 and 24 months, FEV1, FVC, TLC, and DLco improved significantly from earlier declines post-BMT; however FEV1 and FVC remained significantly below pretransplant values. At a median follow-up of 5.5 (1.6-11.6) years, 58 of 110 (53%) patients were surviving. The majority of the patients who died from transplant-related complications suffered from I or more PCs (31/32, 97%). Early PC was associated with over 4-fold reduction in probability of survival at 10 years (8/44, 18% with early PC versus 50/66, 76% without early PC). On multivariate analysis, risk of death was significantly associated with high-risk disease status (P.0 15; hazard ratio [HR] = 2.5), unrelated donor (P = .03; HR = 2.1), early PC (P = .0001; HR = 7.7) and pathogen identification (P = .02; HR = 2.7). These results suggest that, in children undergoing allo-BMT (1) compromised pretransplant lung function is significantly correlated with risk of early respiratory failure but not of overall survival (OS), (2) reductions in lung volumes and diffusion capacity are common 3- to 6-month posttransplant with partial recovery by 12 to 24 months, (3) there is high mortality following mechanical ventilation, and (4) early PCs are associated with significantly worse OS.