CMV pneumonia is a frequent complication of allogeneic bone marrow transplantation (BMT). It usually appears 23 months following transplantation and is associated with a high mortality rate. The incidence of CMV pneumonia in our T-lymphocyte depleted allogeneic BMT recipients, transplanted between 1987-1991, was 18 out of 197 (9.2%) patients. In 3 patients (1.5% of allogenic BMT recipients), pneumonia occurred prior to marrow engraftment, on days 12-16 post BMT. These patients did not develop acute GVHD in contrast to 9/11 patients who had acute GVHD in addition to developing CMV pneumonia between engraftment and day +100 (p < 0.03). Furthermore, these three patients did not receive steroid therapy as opposed to 14/15 patients who were treated with steroids and eventually contracted CMV pneumonia post-engraftment (p < 0.01). The three patients did not have two additional risk factors known for the development of CMV pneumonia: increasing age and a diagnosis of acute myeloblastic leukemia (AML) as the primary disease. Despite prompt diagnosis and therapy with ganciclovir and high doses of intravenous immunoglobulin (IVIG), two of the patients died. Tcell depleted BMT may be a risk factor for development of CMV pneumonia occurring prior to engraftment. In the era of post-BMT anti CMV prophylaxis, one should be aware that life-threatening CMV pneumonia may appear prior to engraftment and consider aggressive CMV prophylaxis.