We aimed to evaluate the frequency and determinants of restless legs syndrome (RLS) in a group of 76 patients with amyotrophic lateral sclerosis (ALS) and 100 control subjects. A diagnosis of RLS was made according to the criteria of the International RLS Study Group, and severity was assessed by the RLS severity scale. RLS was significantly more frequent in patients with ALS (ALS/RLS+) than in control subjects (25% vs. 8%; P = 0.002). Compared with control subjects, patients with ALS/RLS+ showed shorter history of RLS complaints and higher frequency of symptoms occurrence. Moreover, compared with those without RLS, patients with ALS/RLS+ showed increased functional impairment and more often reported sleep complaints. Multivariate logistic regression confirmed the association between RLS and functional impairment. Our findings suggest that RLS should be considered as a possible cause of disrupted sleep in patients with ALS and should be specifically investigated in these patients. © 2010 Movement Disorder Society.