KL-6 is a high-molecular-weight mucinous glycoprotein discovered as a pulmonary adenocaricinoma related antigen. Its levels are used as a biomarker of lung injury in interstitial pneumonia. We here report a case of multiple myeloma with Bence-Jones lambda type whose serum KL-6 level was revealed high at a concentration of 19,400 U/ml. Next, we analyzed the blood test profiles and the concentrations of KL-6 in 20 patients with multiple myeloma. CD19/CD56 double negative fraction on myeloma cells with high expression of CD38 was found in all 5 patients with multiple myeloma having elevated KL-6 level. Patients with interstitial pneumonia show high level of KL-6. We, therefore, need to differentiate the interstitial pneumonia and the above-mentioned multiple myeloma when serum KL-6 level is high.