Immunoresistance (Ab+) to botulinum toxin type A (BTX-A) has been a serous concern since the introduction of BTX-A in the treatment of dystonia and other disorders associated with abnormal muscle contractions. We studied seven patients who developed Ab+ and later reverted to antibodynegative (Ab-) status. These seven patients, six women (mean age, 56 years; range, 41–80 years), with an average duration of dystonia for all patients of 197 months (range, 84–360 months), received a total mean cumulative dose of 1659 units (U) (range,810-1975 U), with an average dose of 207 U per visit. All of these patients became unresponsive to BTX-A treatment and became Ab+ as determined by mouse bioassay. Their response to BTX-A after they revertedd to Ab- was analyzed. The average latency between the initial BTX-A treatment and development of Ab+ was 27 months (range, 15–43 months). The average duration between the detection of Ab+ status and subsequent reversal to Ab- status was 30 months (range, 10–78 months). Six of these Ab- patients were reinjected with BTX-A, and all six benefited from repeat injections comparable with their earlier response. Three patients lost their clinical response to subsequent injections and were found to be again Ab+. Two of the five patients who became immunoresistant to BTX-A received botulinum toxin type F (BTX-F) injections and one patient received a single session of BTX-B with improvement in their symptoms. In conclusion, this unique group of patients who were Ab+ and became Ab- responded favorably to repeat BTX-A injections, but some lost the benefit with subsequent injections. These observations suggest that the anamnestic immunologic response to BTX-A can wane, but can be reactivated by repeat BTX-A treatments. The presence of antibodies did not interfere with the response to BTX-F or BTX-B injections, thus confirming the antigenic specificity of various BTX serotypes.