This study explored relationships between insomnia (as measured by Item 13 of the Clinician-Administered PTSD [Posttraumatic Stress Disorder] Scale) and other PTSD symptoms, comorbidity, and measures of depression, worry, and panic in a mixed-trauma sample of 143 treatment-seeking civilians with PTSD (median age 39.4 years, 83% female, 96% White). Regression analyses showed that, adjusting for sleep medication use, severity of nightmares and diminished interest in pleasurable activities accounted for unique variance in insomnia severity level. Severity of nightmares and depression accounted for unique variance in the presence of severe insomnia. Findings support a role for conditioning related to nightmares in the etiology of PTSD-related insomnia.