Low plasma levels of cortisol, as a result of primary or secondary adrenal hypofunction, is usually associated to findings of hypocortisolism. Low plasma cortisol levels without clinical manifestations have been observed in patients in which cortisol binding capacity (CBG) is altered. In this paper we report a clinically normal patient, with very low plasma cortisol levels. We investigated a possible CBG defect or the presence of a CBG binding substance as an explanation of the clinical findings. We found a normal CBG and no competing substance. Therefore, the presence of low cortisol levels suggests the presence in this patient of a receptor with increased sensitivity to cortisol, or another molecule that can be recognized as glucocorticoid by the cortisol receptor.