Systems and methods for effecting bariatric procedures are disclosed. Each system includes an instrument, a control valve and, optionally, a suction controller. The instrument is in the form of an elongated, flexible sizing tube having a distal end portion including a plurality of apertures. The control valve enables suction to be applied to the patient's stomach via the apertures to drain gastric fluids and to bring adjacent portions of the patient's stomach into engagement with the sizing tube to provide a visually perceptible delineation line along which a portion of the stomach may be resected, sealed and tested.