In familial hyperaldosteronism type I, inheritance of a hybrid 11beta-hydroxylase/aldosterone synthase gene leads to ACTH-regulated overproduction of aldosterone (causing hypertension) and of "hybrid" steroids, 18-hydroxy- and 18-oxo-cortisol. To determine whether complete suppression of the hybrid gene is necessary to normalize blood pressure, we sought evidence of persisting expression in eight patients who were rendered normotensive for 1.3-4.5 yr by glucocorticoid treatment. At the time of the study, six patients were receiving dexamethasone (0.125-0.25 mg/day) and two patients were taking prednisolone (2.5 or 5 mg/day). Urinary 18-oxo-cortisol levels during treatment demonstrated close correlation with mean "day curve" (blood collected every 2 h for 24 h) cortisol (r = 0.74), consistent with regulation by ACTH. Although urinary 18-oxo-cortisol levels were lower during than before treatment (mean 12.6 +/- 2.4 SEM vs. 35.0 +/- 5.6 nmol/mmol creatinine; P < 0.01), they remained above normal (0.8-5.2 nmol/mmol creatinine) in all eight patients. Although mean upright plasma potassium levels during treatment were higher, aldosterone levels lower, PRA levels higher, and aldosterone to PRA ratios lower than before treatment, PRA levels were uncorrected (< 13 pmol/L x min) and aldosterone to PRA ratios were uncorrected (>65) during treatment in four patients. For each of the eight patients, day curve aldosterone levels during treatment correlated more tightly with cortisol (mean r for the eight patients, 0.87 +/- 0.05 SEM) than with PRA (mean r = 0.36 +/- 0.10 SEM). Hence, control of hypertension by glucocorticoid treatment was associated, in all patients, with only partial suppression of ACTH-regulated hybrid steroid and aldosterone production. Normalization of urinary hybrid steroid levels and abolition of ACTH-regulated aldosterone production is not a requisite for hypertension control and, if used as a treatment goal, may unnecessarily increase the risk of Cushingoid side effects.