Objective: Patients with Chronic Low Back Pain (CLBP) show arousal, attentional and cognitive disturbances. The sleep state-dependent P50 midlatency auditory evoked potential was used to determine if patients with CLBP [with and without co-morbid depression (DEP)] show quantitative disturbances in the manifestation of the P50 potential. Methods: P50 potential latency, amplitude and habituation to repetitive stimuli at 250, 500 and 1000 ms interstimulus intervals (ISIs) was recorded, along with the McGill Pain Questionnaire-Short Form (MPQ-SF). CLBP subjects (n=42) were compared with Controls (n=43), and with subjects with DEP only (n=6). Of the CLBP subjects, 20/42 had clinical depression (CLBP+DEP); 8/20 were taking antidepressant medication (CLBP+DEP+med), the others were not (CLBP+DEP-med). Results: There were no differences (ANOVA) in age, sex or P50 potential latency, although there was a trend towards increased latencies in CLBP groups. P50 potential amplitude was lower in CLBP groups, but not in sub-groups, again indicating a trend. P50 potential habituation was decreased in the DEP only subjects at the 250 m ISI, and decreased in CLBP+DEP-med subjects at the 500 ms ISI. This difference was not present in CLBP+DEP+med subjects. The MPQ-SF revealed that patients with CLBP and CLBP+DEP-med showed lower pain scores than CLBP+DEP+med patients. Conclusions: There is decreased habituation of the P50 potential habituation in unmedicated patients with CLBP+DEP compared to Controls. Significance: Patients with CLBP+DEP-med may be less able to disregard incoming sensory information, including painful sensations, but anti-depressant medications help correct this deficit. However, their perception of pain may be increased by medication.