Serum and salivary cortisol, plasma ACTH and serum TSH, free T4, free T3, PRL, LH and FSH were measured before and hourly after the morning steroid replacement therapy of five patients with Addison's disease. In the 4 h after hydrocortisone the levels of TSH fell in all five patients, the levels (mIU/l) decreasing from 6.8 to 3.8; 0.9 to 0.6; 24.3 to 13.2; 28.1 to 7.4 and 17.2 to 11.6. No such change was seen when the procedure was repeated with the morning steroid dose withheld. In two patients, dexamethasone produced similar changes in serum TSH to those observed with hydrocortisone. Steroid therapy had no consistent effect on LH, FSH and PRL levels. Our results show that the time interval between taking medication and withdrawing blood samples should be considered when interpreting TSH levels in treated Addison's disease.