Compared to normal control subjects, patients with primary restless legs syndrome (RLS) have an increased number of K-complexes, followed by bursts of electroencephalographic (EEG) alpha activity-together these are referred to as K-alpha complexes. In the present study, a large proportion of K-alpha complexes was found to be temporally associated with periodic leg movements in sleep. Because many K-alpha complexes precede the onset of movements, they cannot be considered secondary to the movements themselves. The persistence of K-alpha complexes after suppression of leg movements by means of L-DOPA further supports the hypothesis that these EEG events are a primary phenomenon. The results may have clinical implications because a large proportion of RLS patients complain of non-restorative sleep even after treatment with L-DOPA.