A device (1) for monitoring the transverse cross-section of a gastric sleeve (3) as the gastric sleeve (3) is being formed in a stomach (4) during a sleeve gastrectomy procedure comprises a catheter (15) with a primary balloon (19) located thereon towards a distal end (17). The primary balloon (19) is located in the stomach, and the portion of the stomach which is to form the gastric sleeve (3) is stretched around the primary balloon (19) when the primary balloon (19) has been inflated to a diameter approximating to the diameter to which the sleeve (3) is to be formed. As the stomach (4) is being sutured to form the sleeve (3) the pressure within the primary balloon (19) is monitored to avoid excessive stretching of the stomach around the primary balloon (19). The diameter of the primary balloon (19) is also monitored to avoid necking of the sleeve (3) during suturing of the stomach (4).
