Background: Pleurodesis fails in 10%–40% of patients with recurrent malignant pleural effusions malignant pleural effusion and dyspnea. This study aimed to assess the values of pleural elastance (PEL) after the aspiration of 500 mL of pleural fluid and their relation to the pleurodesis outcome, and to compare the pleurodesis outcome with the chemical characteristics of pleural fluid. Methods: A prospective study was conducted in Kasr El-Aini Hospital, Cairo University, during the period from March 2019 to January 2020. The study population consisted of 40 patients with malignant pleural effusion. The measurement of PEL after the aspiration of 500 mL of fluid was done with “PEL 0.5” (cm H2O/L), and the characteristics of the pleural fluid were chemically and cytologically analyzed. Pleurodesis was done and the patients were evaluated one month later. The PEL values were compared with pleurodesis outcomes. Results: After 4-week of follow-up, the success rate of pleurodesis was 65%. The PEL 0.5 was significantly higher in failed pleurodesis than it was in successful pleurodesis. A cutoff point of PEL 0.5 >14.5 cm H2O/L was associated with pleurodesis failure with a sensitivity and specificity of 93% and 100%, respectively. The patients with failed pleurodesis had significantly lower pH levels in fluid than those in the successful group (p<0.001). Conclusion PEL measurement was a significant predictor in differentiating between failed and successful pleurodesis. The increase in acidity of the malignant pleural fluid can be used as a predictor for pleurodesis failure in patients with malignant pleural effusion.