Transudative pleural effusions develop because the distribution of hydrostatic and oncotic pressure across the pleura is altered, so that the rate of pleural fluid formation exceeds that of its reabsorption. They are characterized by a low cell and protein content. Congestive heart failure is the most common cause of transudative effusion. The fluid that accumulates in a hepatic hydrothorax, urinothorax, during peritoneal dialysis, and in many patients with nephrotic syndrome may also have the characteristics of a transudate. The development of a transudative effusion indicates that the pleural membranes per se are intact, so that if the underlying problem can be corrected, the effusion will be reabsorbed.