In order to better characterize the similarities in and differences between the nature of the affective disturbance associated with Posttraumatic Stress Disorder (PTSD) and with Major Depressive Disorder (MDD), self-reported mood and anxiety ratings were examined in PTSD subjects, MDD subjects, and subjects without a psychiatric disorder while they were undergoing a chronobiologic study. Based on serial ratings on visual analogue scales over a 24 hr period, PTSD subjects showed comparable levels of depression as the MDD group, as measured by the mean and maximum levels of mood; however, they had greater mood variability, as measured by the range and coefficients of variation of the mood ratings. The MDD but not the PTSD group had significantly lower mood variability than the non-psychiatric group, as measured by the coefficients of variation. The PTSD group reported higher levels of anxiety than the non-psychiatric or MDD group but showed no differences in any measure of variability of anxiety. These findings suggest there are phenomenologic differences in the affective symptoms experienced by patients with PTSD and with MDD and that mood variability may distinguish between them.