Participants (N=716) were, on average, aged 43 years, female (85%), with a family history of migraine (63%); 52.1% (n=373) completed the study. At baseline, participants reported a mean (SD) of 22.0 (4.8) headache days per month. The proportions of participants with a ≥25%, ≥50%, ≥75%, or 100% reduction in mean monthly headache days continued to improve across the 108 weeks with subsequent onabotulinumtoxinA treatment. Consistent and progressive reductions in headache day responder rates were observed in the analysis of all participants and just those who completed all treatment cycles. In participants who completed all treatment cycles, 75% responders improved from 18.2% at week 24 to 37.7% at week 108. OnabotulinumtoxinA was generally well tolerated, with no new safety signals identified.