Purpose: Cataract surgery on eyes with shallow anterior chambers may be demanding. Glycerol intake prior to surgery has been a well-known method in an effort to increase anterior chamber depth. It is used since it is thought that glycerol as an osmotic agent causes the vitreous body to shrink, pulling back the iris and thereby deepening the anterior chamber – making the surgery easier. Our controlled clinical trial tests this hypothesis and investigates the effect of glycerol on anterior chamber depth (ACD), intraocular pressure (IOP), corneal thickness (CCT), pupil diameter change after viscodilation (PD), operating time and perioperative complications. Methods: We performed a controlled clinical trial. All patients underwent cataract surgery on both eyes with at least 7 days apart. Preoperatively the patient was given glycerol orally when the right eye was operated – when the left eye was operated, nothing was given. In this way, each patient was serving as its own control. Measurements of ACD, IOP and CCT were performed before and after glycerol intake, pupillary diameter was measured before and after viscoelastics during the operation, and operating time and surgical complications were noted. Results: The study included 22 patients with bilateral cataract and anterior chambers depth <2.5 mm. Glycerol caused the anterior chamber to increase by 0.022 mm (p < 0.05), and IOP was lowered by 5.1 mmHg compared to the control group (p < 0.05). However, exposure to glycerol showed no effect on CCT, pupillary dilation of viscoelastics, operating time or surgical complications. Conclusion: Glycerol increases anterior chamber depth and lowers intraocular pressure significantly. These changes had no significant impact on operating time nor on the complication rate, suggesting that these changes are too subtle to have a clinical impact on the cataract procedure.