Objective: To investigate clinical value of serum KL-6 for lung diseases in patients with polymyositis and dermatomyositis. Methods: 77 cases of PM/DM patients, among them 27 cases with interstitial lung disease (ILD), 20 cases with infectious lung disease, 30 cases without lung disease. ELISA method was adopted to measure the KL-6 levels of serum. The association with KL-6 and diagnosis, clinical features, laboratory indexes, treatment, even and prognosis were analyzed. Results: The average values of the KL-6 levels of serum in PM/DM patients with ILD, PM/DM patients with infectious lung disease, and PM/DM patients without lung diseases were (1135.33 +/- 648.67), (371.85 +/- 187.14), (231.00 +/- 110.89), respectively. PM/DM patients with ILD had significantly higher median KL-6 levels of serum compared with other two groups, and the difference is statistically significant (P < 0.05), but the difference between PM/DM patients with infectious lung disease and PM/DM patients without lung disease had no statistical significance (P = 0.229); While the 507 U/ml was regarded as positive standard, the sensitivity of KL-6 for PM/DM interstitial lung disease diagnosis was 96.3%, and the specificity was 89.0%; Serum levels of KL-6 were inversely correlated with DLco%, VC%, and TLV% (all P < 0.05). In serum KL-6 increased group, anti-Jo-1 positive rate was higher than normal group (P < 0.05). A follow-up analysis indicated that, KL-6 levels of serum of 10 cases of PM/DM with ILD patients were associated with therapeutic effect (P < 0.05), and KL-6 levels of serum of 5 cases died of PM/DM with ILD patients were significantly higher than those of 22 cases of PM/DM with ILD patients surviving (P < 0.05). Conclusion: KL-6 levels of serum can be used to identify interstitial lung disease and infectious lung disease, and assess clinical response to treatment, and a high level of KL-6 possibly indicates a poor prognosis.