Plasma renin activity (PRA) and plasma volume (PV) were determined in 22 adult patients treated for Addison's disease (AD) and reporting at the clinic for follow-up. Mean PRA was thrice the upper limit of normal (9.1 +/- 7.1 ng/ml/h (SD)) and mean PV was decreased (87% +/- 11 (SD)), consistent with residual hypovolemia in most patients, despite conventional treatment with both fluorocortisol (FF) and cortisone acetate. There was an inverse relationship between PRA and PV. Both PRA and PV were significantly correlated with FF dosage. On the other hand, no correlation was found between PV and either systolic or diastolic blood pressure (BP), while PRA was significantly correlated with systolic but not diastolic BP. Four patients were persistently hypertensive (diastolic BP greater than 100 mmHg) with elevated PRA in 3, associated with a definitely low PV in two cases. Two of these patients were progressively taken off FF, so as to control BP. Thus, in view of the not infrequent occurrence of arterial hypertension in AD patient on conventional treatment, we would warn against attempts at normalizing PV and PRA by means of FF, irrespective of BP in asymptomatic cases. In fact, when hypertension develops, reduction or sometimes withdrawal of FF may be recommended as a first therapeutic step.