Abstract: An improved, four-chamber chest drainage system include (1) a collection chamber for collecting blood and other liquids to be drained from the pleural cavity of a patient, (2) an underwater seal chamber for preventing contamination of the patient's pleural cavity by atmospheric air, (3) a manometer chamber for accurately indicating the negative pressure in the patient's pleural cavity, and (4) a suction control chamber for regulating the amount of negative pressure (or suction) applied to the patient's pleural cavity. The suction control chamber has a flexible diaphragm adapted to flex in response to increased negativity above a predetermined level to block the opening to the suction source until the negativity returns to the predetermined level. A combination vent valve-and-filter assembly permits ready venting of the collection chamber to filtered atmospheric air.