Objective: To assess the ability of BTX-A injections to lower migraine frequency in chronic sufferers. Methods: Two reviewers independently searched PubMed, Google Scholar and Cochrane Library to locate randomized, double-blind, placebo-controlled trials of BTX-A for chronic migraines. Differences were settled via consensus. Data extraction/verification were managed similarly. The primary outcome was baseline-endpoint change in migraine frequency (number/month). Heterogeneity was assessed using c2 and I-squared. Two raters assessed study quality using the Downs-Black scale, with adjudication via consensus. A fixed-effects model combined study results using the standardized mean difference (Cohen's d) in monthly migraine frequency between placebo and BTX-A groups. Results: Nine trials (N=2114; BTX-A=1388, placebo=726; 2059 completed their trials) provided data in 19 study arms and 9 placebo arms. The average age was 43+/-3, duration of illness was 20+/-3 years, average number of migraines was 6.0+/-2.1/month, 84% were females. All c2 were non-significant; all I-suared were 0, suggesting combinability and confirmed using a fixed-effects model. Quality scores averaged 67%+/-4% ("fair"; range:62%-75%). The weighted average treatment effect (Cohen's d) of BTX-A over placebo was -0.05 (CI95%=-0.13, 0.03) when measured 30 days after injection, -0.04 (CI95%=-0.12, 0.04) at 60 days, and -0.04 (CI95%=-0.12, 0.04) at 90 days post-injection. Two comparisons out of 57 were significant; one by Relja after 60 days and one by Vo after 90 days. Controlling for placebo effect and stratifying by dose stratification found no significant effect of BTX-A in reducting migraine frequency per month over saline vehicle. Conclusion: BTX-A used as prophylactic treatment of chronic migraine headaches does not decrease monthly numbers of migraines.