In this randomised, double-blind, placebo-controlled, exploratory, proof-of-concept phase 2 trial, patients aged 18–55 years with five to 14 migraine days per 28-day period were randomly assigned (1:1) via an interactive web response system to receive an intravenous dose of ALD403 1000 mg or placebo. Site investigators, patients, and the sponsor were masked to treatment allocation during the study. The primary objective was to assess safety at 12 weeks after infusion. The primary efficacy endpoint was the change from baseline to weeks 5–8 in the frequency of migraine days, as recorded in patient electronic diaries. Patients were followed up until 24 weeks for exploratory safety and efficacy analyses. Safety and efficacy analyses were done by intention to treat. This study is registered with ClinicalTrials.gov, NCT01772524.