Objective: To determine the protective action of hyaluronidase on peribulbar anesthesia-related diplopia in patients undergoing cataract surgery. Design: Single-center observational case series. Participants: All patients undergoing elective phacoemulsification and intraocular lens implantation under peribulbar anesthesia between February 2001 and January 2003. Methods: We compared the incidence of postoperative diplopia between 2 periods-February 2001 to January 2002 (P1) and February 2002 to January 2003 (P2)-which differed by the presence (P1) or absence (P2) of hyaluronidase in the anesthetic solution. Main Outcome Measures: All patients were examined on the first and fifth postoperative days during both periods. When diplopia was diagnosed, we recorded the characteristics of the patient, peribulbar anesthesia, and diplopia (orthoptic examination, and magnetic resonance imaging in some cases). Results: Seven thousand two hundred five patients were studied. During P1, 3582 patients received peribulbar anesthesia, and no cases of diplopia occurred. During P2, 3623 patients received peribulbar anesthesia, and 27 cases of diplopia occurred (incidence, 0.75%; P=0.0002 vs. P1). Diplopia involved the inferior rectus (40%) and the external rectus (37%) muscles. Diplopia was persistent in 54%of the cases. Conclusions: Peribulbar anesthesia-related diplopia was significantly more frequent when hyaluronidase was not added to the anesthetic solution. Objective: To determine the protective action of hyaluronidase on peribulbar anesthesia-related diplopia in patients undergoing cataract surgery. Design: Single-center observational case series. Participants: All patients undergoing elective phacoemulsification and intraocular lens implantation under peribulbar anesthesia between February 2001 and January 2003 . Methods: We compared the incidence of postoperative diplopia between 2 periods-February 2001 to January 2002 (P1) and February 2002 to January 2003 (P2) -which differed by the presence (P1) or absence (P2) of hyaluronidase in the anesthetic solution. Main Outcome Measures: All patients were examined on the first and fifth postoperative days during both periods. When diplopia was diagnosed, we recorded the characteristics of the patient, peribulbar anesthesia, and diplopia (orthoptic examination, and magnetic resonance imaging in some cases Results: Seven thousand two hundred five patients were studied. During P1, 3582 patients During P2, 3623 patients received peribulbar anesthesia, and 27 cases of diplopia occurred (incidence, 0.75%; P = 0.0002 vs. P1). Diplopia involved the inferior rectus (40%) and the external rectus (37%) muscles. Diplopia was persistent in 54% of the cases. Conclusions: Peribulbar anesthesia-related diplopia was significantly more frequent when hyaluronidase was not added to the anesthetic solution.