KL-6, a mucinous high-molecular weight glycoprotein expressed on type 2 pneumocytes, has been shown to be elevated in the serum and bronchoalveolar lavage fluid of patients with interstitial pneumonitis (IP). We measured the serum levels of KL-6 in patients after they had undergone allogeneic bone marrow transplantation (BMT) to determine whether KL-6 could be a clinically useful indicator for the development of IP. The serum concentrations of KL-6 were determined by a sandwich-type enzyme-linked immunosorbent assay using an anti-KL-6 monoclonal antibody. A total of 1028 samples were tested from 76 patients (78 transplantations) who received BMTs. The KL-6 values were markedly elevated in patients with pulmonary complications, but not in those with acute and chronic graft-versus-host disease, hemorrhagic cystitis, herpes encephalitis, sepsis, and veno-occlusive disease. The serum levels of KL-6 from patients with pulmonary complications were significantly higher than from those without pulmonary complications (P < .001) and those with other complications (P < .001). Of the 12 patients with pulmonary complications, 6 had idiopathic IP (IIP). The levels were not high at the onset of IIP. Four of 6 IIP patients showed marked elevations of KL-6 levels in parallel with the severity of IP and died of respiratory failure without response to treatment. Assessment of serum KL-6 levels might not be useful for the early diagnosis of IP, but may be a useful indicator for monitoring the severity of IP after BMT.