PURPOSE: To evaluate the efficacy and side effects of concentrated versus dilute botulinum toxin A in treating benign essential blepharospasm. METHODS: The authors performed a prospective randomized clinical trial of 16 patients with an established diagnosis of benign essential blepharospasm. Patients were randomized to receive low concentration (control, 10 U/ml) injections on one side and high concentration (experimental, 100 U/ml) injections on the other. They were surveyed on a scale of 1 to 10 regarding pain, bruising, and redness immediately after the injection. During their return visit, at an established interval of 1 to 3 months, patients were questioned regarding complications (ptosis, diplopia, tearing, and dry eye), duration of relief, and side preferred. Patients were followed over 8 months for 1 to 6 repeat injections, with the side given the higher concentration alternated at each visit. RESULTS: With 16 patients, there were a total of 42 visits and 84 observations (eyes) documented. Using the Wilcoxon rank sum test, there was a statistically significant reduction in pain scores (1.94 vs. 4.59, p < 0.001) on the experimental side versus the control side. Patient assessment revealed no significant difference in bruising, redness, complications of injection, side preference, or length of relief of symptoms. CONCLUSIONS: Compared with the control, the high concentration botulinum toxin A demonstrated a 58% reduction in perceived pain. Patients did not report a significant difference in efficacy or complications with either dilution. © 2009 The American Society of Opthalmic Plastic and Reconstructive Surgery, Inc.