When an abnormal amount of fluid accumulates in the space between the 2 layers of the pleura (the membrane that lines the surface of the lungs), it is called a pleural effusion. It is estimated that 1 million people in the United States develop a pleural effusion each year. About 20% to 60% of the people who develop pneumonia have a pleural effusion. A pleural effusion can also occur due to an extrapulmonary (outside the lungs) cause. Treatment depends on the underlying cause. The January 21, 2009, issue of JAMA includes an article about diagnosing pleural effusions.