The incidence of interstitial pneumonitis (IP) was reviewed in 80 consecutive patients who received allogeneic T lymphocyte-depleted bone marrow transplantation (BMT) for malignant and non-malignant diseases. Pretransplant conditioning used in malignant disorders included total lymphoid irradiation (TLI) 150 cGy x 4, total body irradiation (TBI) 200 cGy x 6, and cyclophosphamide (CY) 120 mg/kg. In non-malignant diseases conditioning included no TBI, but adjusted doses of TLI in addition to CY (severe aplastic anemia) or CY and busulfan (severe beta-thalassemia major). In the malignant group only one patient developed graft-versus-host disease (GVHD) grade I; IP developed in 12 out of 61 patients (19.7%) and IP-associated fatality occurred in five patients (8.2%). Cytomegalovirus (CMV) was associated with only two of the five fatal IP. In the non-malignant group there was no GVHD; one patient out of 19 (5.2%) had IP, which was fatal and not associated with CMV. These data indicate that fatal IP may appear in the absence of GVHD. The relatively low incidence of IP-related mortality in recipients of allogeneic T lymphocyte-depleted BMT suggests that although prevention of GVHD and elimination of drugs used for GVHD prevention may reduce the incidence of fatal pulmonary complications, other approaches have to be investigated for complete prevention of IP which still represents a major complication in patients with malignant hematologic disorders treated by allogeneic BMT.