The recommended natalizumab dosage is 300 mg every 4 weeks. We evaluated radiological activity at various times from the last natalizumab infusion by examining 386 magnetic resonance imaging (MRI) scans from 166 natalizumab-treated patients with relapsing-remitting MS. Of 113 scans performed >4 weeks after last natalizumab infusion, 26 were active (i.e. had ≥1 contrast-enhancing lesions). Risk of radiological activity increased by 1.34 fold for each week of delay with respect to the recommended 4-week dosing interval, compared with schedule-adherent patients (p<0.0001). Our data suggest that an increased MRI activity ≥7 weeks from the last infusion of natalizumab should be considered in cases of therapy discontinuation.