Background: This study aimed to clarify the effect of thoracic drainage fluid (DF) on lung cancer cells in vitro. 
Methods: We assessed the influence of DF on the proliferation and migration of lung cancer cells (LTEP-a-2 and A549) using the MTT cell proliferation assay and scratch wound assay. Cell apoptosis was determined by flow cytometric analysis. We also investigated the effect of DF on drug chemosensitivity, assessing viability of LTEP-a-2 and A549 cells. 
Results: The proliferative rates of cancer cells in the DF-treated group were significantly higher than those of the control group. Similar results were obtained for cell migration of lung cancer cells. Cells in the DF-treated groups showed a lower percentage of apoptosis than those of the control groups. Chemosensitivity of lung cancer cells to doxycycline and cisplatin (DDP) was lowered by DF. 
Conclusion: These findings suggest that DF affects lung cancer cells by promoting proliferation and migration, inhibiting apoptosis, and increasing drug resistance.
Keywords: lung cancer, drainage fluid, proliferation and migration, apoptosis, drug resistance