Urinary free-cortisol levels (micrograms per day) were measured by radioimmunoassay at 2-week intervals during the course of hospitalization in the following patient groups: posttraumatic stress disorder (PTSD); major depressive disorder; bipolar I, manic; paranoid schizophrenia; and undifferentiated schizophrenia. The mean cortisol level during hospitalization was significantly lower in PTSD (33.3 +/- 3.2) than in major depressive disorder (49.6 +/- 5.9), bipolar I, manic (62.7 +/- 6.7), and undifferentiated schizophrenia (50.1 +/- 8.9), but was similar to that in paranoid schizophrenia (37.5 +/- 3.9). The same differences across groups are evident in the first sample following hospital admission. This finding of low, stable cortisol levels in PTSD patients is especially noteworthy, first because of the overt signs of anxiety and depression, which would usually be expected to accompany cortisol elevations, and second because of the concomitant chronic increase in sympathetic nervous system activity shown in prior psychophysiological studies of PTSD and reflected in marked and sustained urinary catecholamine elevations previously reported in our own PTSD sample. The findings suggest a possible role of defensive organization as a basis for the low, constricted cortisol levels in PTSD and paranoid schizophrenic patients. The data also suggest the possible usefulness of hormonal criteria as an adjunct to the clinical diagnosis of PTSD.