Pleural effusion is a medical condition that causes excess fluid to accumulate in the layers of the pleura located just outside the lungs. This condition is often referred to as “water on the lungs”. The area known as the pleura, where the fluid accumulates, are thin membranes that lubricate the lungs while breathing. It is normal for very small amounts of fluid to be present in this area, but occasionally it can begin to accumulate, causing pleural effusion.