Pulmonary complications (PC) following HSCT remain a significant source of morbidity and mortality in both the early and late HSCT period. BAL has been a useful initial diagnostic tool in the evaluation of post HSCT PC; however, the sensitivity of BAL versus Lung Bx in pediatric HSCT pts has not been analyzed. We reviewed 208 pediatric HSCT recipients who underwent a total of 252 HSCT. Allo (174) Myeloablative Conditioning (MAC) (54%), Reduced Intensity Conditioning (RIC) (45%)and Auto (78) All MAC. Sixty-six patients (32%) underwent 107 BAL for fever, respiratory distress, and/or pulmonary infiltrate(s) on chest X-ray/CT scan: M:F = 38:28, median age 10 yrs (.33–21), mean # of BAL/pt 2 (1–8). The probability of requiring BAL after HSCT in all pts was 23.7% (CI95:18–29.4); RIC 21% (11.5–30.6) vs MAC 24% (16.8–31.6) p = 0.61 (NS). The median day s/p Auto and Allo at the time of BAL was Day +31 and +94, respectively. The probability of AGVHD Grade II-IV at the time of BAL after Allo SCT was 70% (50–90) and cGVHD was 85% (57–100). The risk of having cGVHD was significantly higher in the BAL vs no BAL group (p < 0.001). Pathogens were identified in 37 BAL (35%): Bacteria (16), Fungus (12), Viral (13). Seventeen of the 208 pts (8%) underwent a total of 21 lung biopsies. Nineteen (90%) of the biopsies identified at least one etiology: Infection (12), Fibrosis (4), GVHD (5), recurrent malignant disease (1). Of the 9 biopsies that were performed within 30 days of an inconclusive BAL (median 13 days s/p BAL), 6 (67%) identified at least one infection pathogen: Aspergillus (1), CMV (1), Parainfluenza/Influenza (1), Parainfluenza 3 (1), MAC (1), HSV-1 (1). There were no Grade III/IV toxicity secondary to BAL or Bx. Lung autopsies were performed in thirteen pts (20%) whom underwent a negative BAL. Five (38%) provided at least one pathogen previously unidentified via BAL performed within 30 days: Staph Epi (1), Adenovirus (2), CMV (2), Candida Glabrata (1). The overall survival (OS) in pts whom required BAL and lung Bx was 45% (16–75) vs 76% (69–83%, p < 0.003) in the no BAL and no Bx group This data suggest that a BAL post HSCT in pediatric recipients will likely only yield a Diagnosis (Dx) 35% of the time compared to lung Bx which had a 90% diagnostic yield. In the future a paradigm will be required to determine which pediatric HSCT recipients may benefit by having an initial lung Bx and foregoing an initial BAL to Dx PC.