In a model of idiopathic pneumonia syndrome after bone marrow transplantation (BMT), injection of allogeneic T cells induces nitric oxide (·NO), and the addition of cyclophosphamide (Cy) generates superoxide (O·) and a tissue-damaging nitrating oxidant. We hypothesized that ·NO and O· balance are major determinants of post-BMT survival and inflammation. Inducible nitric oxide synthase (iNOS) deletional mutant mice (−/−) given donor bone marrow and spleen T cells (BMS) exhibited improved survival compared with matched BMS controls. Bronchoalveolar lavage fluids obtained on day 7post-BMT from iNOS(−/−) BMS mice contained less tumor necrosis factor-α and interferon-γ, indicating that ·NO stimulated the production of proinflammatory cytokines. However, despite suppressed inflammation and decreased nitrotyrosine staining, iNOS(−/−) mice given both donor T cells and Cy (BMS + Cy) died earlier than iNOS-sufficient BMS + Cy mice. Alveolar macrophages from iNOS(−/−) BMS + Cy mice did not produce ·NO but persisted to generate strong oxidants as assessed by the oxidation of the intracellular fluorescent probe 2′,7′-dichlorofluorescin. We concluded that ·NO amplifies T cell-dependent inflammation and addition of Cy exacerbates ·NO-dependent mortality. However, the lack of ·NO during Cy-induced oxidant stress decreases survival of T cell-recipient mice, most likely by generation of ·NO-independent toxic oxidants.