Apparatus (135) monitors the internal transverse cross-section of a stoma (120) in a stomach (121) being formed by a gastric band (122). The apparatus comprises a balloon catheter (65) having a catheter 2 and a balloon (6) mounted thereon. Measuring electrodes (15,16) are located on the catheter (2) within the balloon (6) for producing signals indicative of the cross-sectional area of the stoma (120) when the balloon (6) is located in the stomach (121) adjacent the stoma (120) as the balloon (6) is being inflated with an electrically conductive inflating medium by a first pump (39). A second pump (140) is provided for inflating a cuff (127) of the gastric band (122) for altering the tightness with which the stoma (120) is formed by the gastric band (122). A signal processing and control apparatus (36) determines the cross-sectional area of the stoma (120) from signals received from the measuring electrodes (15,16), and controls operation of the second pump (140) for controlling the tightness of the gastric band (122) around the stomach (121) in response to the values of the transverse cross-sectional area of the stoma (120) to produce the stoma (120) to be of a desired internal transverse cross-section.
