Objective: To explore the clinical significance of detecting epidermal growth factor receptor (EGFR) mutations in hydrothorax of patients with non-small cell lung cancer (NSCLC). Methods: The mutations of EGFR in exon 18-21 were detected with Sanger sequencing in 17 hydrothorax samples and 17 tumor tissue samples in parallel from patients with NSCLC. Results: There were 5 cases (29.41％) with EGFR mutations in 17 hydrothorax samples and 7 cases (41.18％) in 17 tumor tissue samples. The detection rate of EGFR mutations in hydrothorax samples was lower than that in tumor tissue samples. Conclusion: The hydrothorax sample can be used to detecting EGFR mutations by Sanger sequencing. And it is helpful to patients with NSCLC who can't obtain tumor tissue.