Obliterative bronchiolitis (OB) after lung transplantation is the end result of multiple immunologic, virologic, genetic, and environmental effects on the transplanted lung. In this study, we first analyzed risk factors for OB in a single-center population of 152 lung transplant recipients. We then examined the influence of donor and recipient HLA mismatching on progression to OB, and on the identified risk factors for OB. The median time to onset of OB for the entire study population was 2.7 yr. The significant risk factors for OB by multivariate analyses were grade A2 or A3 acute rejection (p = 0.0126) and cytomegalovirus (CMV) pneumonitis (p = 0.0358). The only significant HLA risk factor for OB was mismatching at the HLA-A locus (p = 0.0144). On the basis of Cox proportional hazards modeling, a predictive formula was derived to estimate the risk of OB after lung transplantation. Although mismatching at the HLA-DR locus was a significant risk factor for CMV pneumonitis in recipients exposed to CMV before transplantation (p = 0.0199), and protected against acute rejection, it did not independently protect against OB. These results indicate that HLA mismatches between donors and recipients significantly influence the development of OB both directly, and indirectly, by influencing the major risk factors for OB.