Increased morning plasma free cortisol levels have been reported previously in chronic renal failure (CRF) patients. To see whether binding abnormalities of plasma proteins contributed to the increase in morning free cortisol, binding characteristics of corticosteroid-binding globulin (CBG) were studied in pooled plasma from CRF patients and normal subjects. Using an isocolloidosmolar equilibrium dialysis method the unbound, albumin-bound, and CBG-bound fractions of plasma cortisol were measured within the physiologic range of plasma cortisol levels. Cortisol binding to CBG was equal in uremic and nonuremic plasma as determined by the affinity constants and the binding capacities of CBG for cortisol. In addition, the distribution of cortisol binding was identical in uremic and in nonuremic plasma samples at equal total cortisol concentrations. By confirming that cortisol binding properties of CBG are unaltered in CRF patients, these data support the assertion that elevated morning free cortisol levels in CRF patients are probably due to increases of plasma total cortisol.