OBJECTIVE: It is well known that the serum level of KL-6 can be an indicator of disease activity in patients with interstitial pneumonia (IP). However, surgical lung biopsy is often required for the diagnosis of IP, although this can result in IP exacerbation. METHODS: The effect of surgical lung biopsy on the serum level of KL-6 in patients with IP was analyzed. Thirty-two cases of IP were examined in this study. There were no cases showing exacerbation of IP. RESULTS: The serum level of KL-6 demonstrated 1067+/-550 U/ml (mean+/-SD) before lung biopsy, 991+/-471 U/ml a day, 824+/-377 U/ml 4 days and 826+/-384 U/ml 7 days after lung biopsy. The serum KL-6 levels on the 1st, 4th, 7th day after the lung biopsy were significantly lower than that before the lung biopsy (P<0.05, P<0.01 and P<0.01, respectively). The percent decrease of the serum KL-6 levels on the 4th day (the lowest level) was dependent on the urine volume, and the analysis of the urinary levels of KL-6 showed a transient increase in urinary KL-6 excretion, suggesting that the decrease in serum KL-6 levels associated with surgical lung biopsy may be caused by this increase in urinary KL-6 excretion. CONCLUSION: Surgical lung biopsy of patients with IP has little effect on the increase in serum KL-6 levels. An elevation of serum KL-6 after surgical lung biopsy may indicate exacerbation of IP.