Idiopathic pneumonia syndrome (IPS) is a significant cause of morbidity and mortality after bone marrow transplantation (BMT) in humans. We developed a murine IPS model in which lethal pre-BMT conditioning and allogeneic T cells results in the recruitment of host monocytes and then donor T cells into the lung by day 7 after BMT, concomitant with development of severe lung dysfunction. We reported the T cell–dependent production of the T cell–attracting chemokine macrophage inflammatory protein-1α (MIP-1α) in the lungs of such recipient mice. We reasoned that MIP-1α might be a critical mediator of IPS. Lethally conditioned mice received transplants of major histocompatibility complex–disparate marrow and either wild-type (MIP-1α+/+) or knockout (MIP-1α−/−) spleen cells. Recipients of MIP-1α−/− cells exhibited accelerated mortality and a decrease in specific compliance that appeared earlier than in recipients of MIP-1α+/+ cells. Donor CD4+ and CD8+ T cell expansion was increased in the spleens of recipients of MIP-1α−/−cells. Lungs of recipients of MIP-1α−/− cells had earlier recruitment of both T-cell subsets by day 3 after BMT, concomitant with the influx of cells expressing the cytolysins granzymes A and B. Monocyte recruitment was not altered. Levels of inflammatory cytokines were not increased and levels of T cell–attracting chemokines were decreased. The level of the anti-inflammatory cytokine interleukin 13 (IL-13) was lower in the serum and lungs of recipients of MIP-1α−/− cells, indicating a skewing toward a more inflammatory T helper cell type 1 (Th1) cytokine milieu. Donor-derived MIP-1α may play a role in allogeneic-induced IPS by limiting aggressive expansion of CD4+ and CD8+ T cells.