We have studied a woman with an apparent receptor-mediated resistance to cortisol on the basis of elevated 24-h mean plasma cortisol levels and increased urinary free cortisol. Plasma ACTH concentrations were normal but she was resistant to adrenal suppression by dexamethasone. No stigmata of Cushing's syndrome were seen. To study the proposed end-organ resistance to cortisol, we examined the glucocorticoid receptor (GR) in lymphocytes and in fibroblasts from this patient and from her son. Several molecular properties of the GR of lymphocytes from the patient were indistinguishable from that of normal control subjects. In thermolability assays, however, the patient's GR as well as her son's GR showed a striking heat sensitivity at 40 degrees and 45 degrees C when compared with GR from normal persons. In addition, data from the thermolability assays correlated well with the lack at 45 degrees C of dexamethasone-induced decrease in in vitro [3H]thymidine incorporation into lymphocytes derived from both patients.