PURPOSE: A small percentage of cases with essential blepharospasm or hemifacial spasm will become resistant to botulinum toxin A (Botox). We present our experience treating these patients with botulinum toxin B (Myobloc). METHODS: We reviewed all charts of patients in one physician's practice who received botulinum toxin B after becoming refractory to botulinum toxin A. For each treatment session, patients were evaluated for side effects, relief of spasms, and duration of treatment effect. RESULTS: Data were collected on 16 patients and a total of 93 treatment visits. Average total dosage was 3,633 U per treatment session. Mean duration of beneficial effect was 7.3 weeks and was most commonly rated as fair to excellent. Side effects for botulinum toxin B occurred at a higher rate than is typical for botulinum toxin A. Most common side effects were pain on injection (100%), ptosis (32.3%), facial or mouth droop (22.6%), dry mouth (17.2%), dry eye (9.7%), exposure keratitis (7.5%), and blurred vision (5.4%). The rate of occurrence of side effects appears to be dose-dependent. CONCLUSIONS: Botulinum toxin B is a useful treatment for cases that have become refractory to botulinum toxin A. The duration of beneficial effect is shorter than for type A. The side effect profile is similar to that for toxin type A except for pain on injection and the occurrence of dry mouth. Side effects occurred at a higher frequency than with botulinum toxin A, and the pattern suggests that botulinum toxin B spreads more diffusely. Side effects appear to be dosage-related.