BACKGROUND AND OBJECTIVES: The objective of this study was to compare the quality and safety of retrobulbar block for the treatment of cataracts by phacoemulsification using needles of different sizes: 27 x 0.70 mm and 20 x 0.55 mm. METHODS: Candidates for ophthalmic block for the treatment of cataracts by phacoemulsification were selected prospectively. After sedation and standardized monitoring, a retrobulbar block with 4 mL of 1% ropivacaine containing hyaluronidase was performed with a single inferolateral puncture. In Group I (GI), the 25 x 0.70 mm needle was used, while in Group II (GII) a 20 x 0.55 mm needle was used. All patients had a CT scan at the time of the blockade and five minutes after the administration of the anesthetic solution with radiologic contrast. Ocular mobility was assessed after three, five, and ten minutes. The incidence of complications in both groups and the satisfaction of patients and surgeons were compared by applying a standardized questionnaire. RESULTS: Fifty-four adult patients were evaluated (27 per group). Patients did not complain of intraoperative pain or perception of light. The anesthetic procedure was considered very good (VG) or excellent (E) in 96.3% of the cases in both groups. All patients were subjected to the same assessment. The need of block repetition between both groups did not differ (p = 1.0). Patients in GI showed decreased ocular mobility at 3 minutes (p = 0.03). The intraconal retrobulbar placement of the needle was observed on both groups. Anesthetic dispersion at 5 minutes was similar in the two groups. CONCLUSIONS: Retrobulbar block with a single inferolateral puncture with a 20 x 0.55 mm needle, using low doses of anesthetics, is an effective and safe option for the treatment of cataracts by phacoemulsification.