Hepatic veno-occlusive disease (HVOD) and interstitial pneumonitis (IP) are both widely regarded as toxicities of intensive cytoreductive therapy, but their association has not been previously examined. Risk factors for IP were evaluated in 154 patients given intensive cytoreductive therapy followed by allogeneic bone marrow transplantation during a 2 1/2 year period. IP occurred in 68 patients; HVOD occurred in 39. The actuarial incidence of IP in patients with VOD was 71% and 45% in those without VOD (p = 0.0002). In multivariate analysis, the diagnosis of hematologic malignancy (p less than 0.001), the occurrence of HVOD (p less than 0.01), and pretransplant CMV seropositivity (p less than 0.02) were significantly associated with IP. The individual relative risks for IP of presence to absence of these factors was 4.5 for the diagnosis of hematologic malignancy, 2.1 for HVOD, and 1.9 for CMV seropositivity. Pulmonary veno-occlusive disease (PVOD), a previously rare observation, was noted at autopsy in 1/5 (20%) patients with HVOD alone, 6/20 (30%) patients with IP alone, and 10/14 (71%) of patients with both HVOD and IP. The association of HVOD and IP is supportive of the concept that toxic effects of cytotoxic therapy have a major role in pathogenesis of HVOD and IP.