Abstract: Objective To comparatively analyze the X-ray signs with the CT findings of 280 cases in hydrothorax so as to evaluate the value of CT in the diagnosis of hydrothorax. Methods 280 cases of hydrothorax were examined with fluoroscopy, X-ray anterioposterior and lateral film and CT scanning. Results The signs of X-ray were compared with the specific symptoms of CT. Conclusion We can diagnose precisely if the hydrothorax is caused by the inflammatory lesion or benign and malignant tumor.