To determine the sensitivity of serum KL-6 and serum lactate dehydrogenase for detecting the contraction of radiation pneumonitis, 15 patients with lung cancer who had radiation therapy were monitored. Six of the patients contracted radiation pneumonitis (pneumonitis group) and the other patients did not (control group). Serum levels of KL-6 were significantly (p less than 0.05) elevated according to the complication of radiation pneumonitis in all patients of the pneumonitis group. In the control group, however, one-sided changes of KL-6 level were not observed. In the pneumonitis group, serum LDH levels were not significantly changed. However, there was a strong correlation between the altered levels of KL-6 and those of LDH (r = 0.992). These observations indicate that the same cytopathologic changes may cause the elevation of serum KL-6 level and the elevated activity of serum LDH in the patients with radiation pneumonitis, and that KL-6 is much more sensitive than LDH for detecting radiation pneumonitis.