Pleural effusion is the accumulation of fluid in the space between the membranes (pleura) surrounding the lungs. There are many different causes for this condition. The causes can be related to the lungs or stem from systemic diseases outside the lungs. Among the most common causes of pleural effusion are infections originating from the lungs or pleura. Furthermore, certain rheumatic diseases, heart failure, traumas, and various types of cancer can also lead to increased fluid in the pleural space. In certain regions, such as Turkey, tuberculosis is a significant factor contributing to pleural effusion or pleural thickening.