Abstract: A drainage device is provided with a collection chamber, an underwater seal chamber, and a separate flow chamber. The underwater seal chamber which prevents the flow of atmospheric air from the device into the pleural cavity of the patient is formed by a recessed portion in a partition separating the underwater seal chamber from the collection chamber. A passageway from the underwater seal chamber into the collection chamber is located in the partition separating the chambers and a baffle is formed around the opening to prevent excessive loss of fluid from the underwater seal chamber into the collection chamber when the device is inadvertently tilted. The flow chamber is closed off from the collection chamber and is connected with the underwater seal chamber by means of a tubular passageway adjacent the upper wall of the drainage apparatus so as to prevent fluid from passing from the underwater seal chamber into the outlet passageway.