 This is a multicenter, double-blind, placebo-controlled, 3 arm, parallel group study of HORIZANT in adolescents 13 to 17 years of age diagnosed with moderate-to-severe primary RLS. Eligible patients enter a 7-day screening period during which safety assessments are performed. Eligible patients are randomized in a 1:1:1 ratio to HORIZANT 300 mg or 600 mg, or matching placebo, followed by a 12-week treatment period. Patients take the study drug once daily at approximately 5 PM with food. Patients will visit the clinical site on 5 or 6 different occasions. 