This Phase 3 trial is an open-label extension study of the placebo-controlled, double-blind VRX-RET-E22-301 trial. Patients who have completed the VRX-RET-E22-301 trial and who meet inclusion and exclusion criteria will be treated with 600-1200 mg/day of retigabine as an adjunct therapy to their current antiepileptic drugs (AEDs) or vagal nerve stimulation. Treatment will be continued until retigabine is commercially available, or until the program is discontinued. Patients will be recruited from 45-50 sites in the United States, Canada, Mexico, Brazil, and Argentina. The safety and tolerability of long-term therapy with retigabine administered as adjunctive therapy in adult epilepsy patients with partial-onset seizures will be evaluated. In addition, the efficacy of long-term treatment with retigabine and patient quality of life will be assessed.