BACKGROUND Nightmares are common in posttraumatic stress disorder (PTSD), but they also frequently occur in idiopathic form. Findings associated with sleep disturbances in these two groups have been inconsistent, and sparse for idiopathic nightmares. The aim of the present study was to investigate whether sleep anomalies in PTSD sufferers with frequent nightmares (P-NM) differ from those observed in non-PTSD, idiopathic nightmare (I-NM) sufferers and healthy individuals. METHODS Sleep measures were obtained from nine P-NM sufferers, 11 I-NM sufferers, and 13 healthy control subjects. All participants slept in the laboratory for two consecutive nights where electroencephalogram, electro-oculogram, chin and leg electromyogram, electrocardiogram, and respiration were recorded continuously. RESULTS Posttraumatic nightmare sufferers had significantly more nocturnal awakenings than did I-NM sufferers and control subjects. Elevated indices of periodic leg movements (PLMs) during rapid eye movement (REM) and non-REM sleep characterized both P-NM and I-NM sufferers. CONCLUSIONS Posttraumatic nightmare sufferers exhibit more nocturnal awakenings than do I-NM sufferers and control subjects, which supports the hypothesis of hyperarousal in sleep in PTSD sufferers; however, elevated PLM indices in both P-NM and I-NM sufferers suggest that PLMs may not be a marker of hyperarousal in sleep of PTSD sufferers. Rather, PLMs may be a correlate of processes contributing to intense negative dreaming.