Objective To investigate the levels of matrix metalloproteinases-3 (MMP-3), adenosine deaminase (ADA) and lactate dehydrogenase (LDH) in the hydrothorax and ascites, and to approach the diagnostic value of three combined indexes in benign and malignant hydrothorax and ascites. Methods Case-control study. A total of 278 patients with hydrothorax and ascites were enrolled in this study who were hospitalized in the First Affiliated Hospital of Hunan University of Traditional Chinese Medicine from August 2018 to July 2019 to detect the levels of MMP-3, ADA and LDH in the hydrothorax and ascites. The benign group (208 patients) and malignant group (70 patients) were compared with MMP-3, ADA, LDH, receiver operating characteristic (ROC) curve, sensitivity and specificity in the hydrothorax and ascites, and the results were compared comprehensively. Results (1)The MMP-3 level in the benign hydrothorax group was 89.21±61.93 ng/mL, the ADA level was (9.08±8.89) U/L, the LDH level was (143.34±68.63) U/L, and the MMP-3 level in the malignant hydrothorax group was (205.63±98.16) ng/mL, he ADA level was (10.96±5.04) U/L, the LDH level was (243.44±131.20) U/L. The MMP-3 level in the benign ascites group was (84.91±73.48) ng/mL, the ADA level was (3.48±2.80) U/L, the LDH level was (99.48±69.53) U/L, and the MMP-3 level in the malignant ascites group was (174.89±82.48) ng/mL, the ADA level was (6.31±4.42) U/L, the LDH level was (191.86±94.52) U/L. The levels of MMP-3, ADA and LDH in the hydrothorax and ascites of the malignant group were higher than those in the benign group, and the difference was statistically significant (Z1 values were 5.215, 2.549, 3.212, respectively, and Z2 values were 6.188, 4.524, 6.38, respectively, P1 and P2 were <0.05). (2)The area under the curve (AUC) of MMP-3 for diagnosis of hydrothorax, liver cancer ascites and gastric cancer ascites was 0.853, 0.826, and 0.763, respectively. The sensitivity was 76%, 96.9%, and 92.3%, respectively, and the specificity was 80%, 64.5%, 61.6%. The diagnostic efficacy of MMP-3 in lung cancer hydrothorax and liver cancer ascites was higher than ADA (AUC were 0.672, 0.691,respectively) and LDH (AUC were 0.717, 0.804, respectively), and the diagnostic efficacy of gastric cancer ascites was lower than ADA (AUC is 0.808) and LDH (AUC is 0.849), and LDH was the best. (3)The AUC of MMP-3, ADA and LDH combined diagnosis of lung cancer hydrothorax, liver cancer ascites and gastric cancer ascites were 0.861, 0.842, and 0.879, respectively. The sensitivities were 64%, 96.9%, and 84.6%, respectively, and the specificities were 92.9%, 63.8%, and 80.4%, respectively. In the lung cancer hydrothorax, liver cancer ascites and gastric cancer ascites, the combined efficacy of the three combined tests was better than the combined detection of MMP-3 and LDH (AUC were 0.86, 0.839, 0.872, respectively), combined detection of MMP-3 and ADA (AUC were 0.845, 0.831, 0.855, respectively), LDH and ADA combined detection (AUC were 0.713, 0.791, 0.846, respectively). Conclusions MMP-3 is important for the differential diagnosis of benign and malignant hydrothorax and ascites, and may be one of the important indicators for the differential diagnosis of benign and malignant hydrothorax and ascites. The diagnostic efficacy of MMP-3 combined with ADA and LDH and three combined detection is better than single index, which has certain clinical value for differential diagnosis of benign and malignant hydrothorax and ascites. Key words: Matrix metalloproteinase 3; Adenosine deaminase; Lactate dehydrogenases; Hydrothorax; Ascites; Diagnosis, differential