PURPOSE Large diurnal intraocular pressure (IOP) fluctuation ([INCREMENT]IOP) is believed to be one of the causes of progression in glaucomatous changes. Some fully medicated glaucoma patients whose IOPs are controlled during the regular office hours (10:00 to 16:00 h) still have progression in glaucomatous changes and IOP elevation during off-office hours. The purpose of this study was to determine whether [INCREMENT]IOP is dampened after combined trabeculotomy and sinusotomy (LOT+SIN) in glaucoma patients with low IOPs during the regular office hours. PATIENTS AND METHODS Fourteen eyes of 8 open-angle glaucoma patients who had large [INCREMENT]IOP despite low IOPs during the office hours were studied. The IOP was measured every 3 hours for 24 hours before and >3 months after the operation. The IOPs were measured in the sitting position with a Goldmann applanation tonometer. All patients underwent LOT+SIN. RESULTS All patients had IOP elevations >20 mm Hg between 0:00 and 3:00 hours before the operation, and none had an IOP peak after the operation. The postoperative mean IOP (16.5±1.7 to 13.9±2.0 mm Hg, P=0.00064), the maximum IOP (21.9±2.4 to 16.1±2.5 mm Hg, P=0.0020), and [INCREMENT]IOP (8.9±2.7 to 4.3±1.2 mm Hg, P=0.0032) were significantly lower than the preoperative values. However, the minimum IOP was not reduced significantly (13.0±1.9 to 11.7±1.7 mm Hg). CONCLUSIONS The diurnal [INCREMENT]IOPs are dampened by LOT+SIN in glaucoma patients with controlled IOPs during regular office hours. These results indicate that these surgical procedures can be used for the treatment of open-angle glaucoma patients.