The prevalence, reversibility, and mortality of secondary hypertension among 3783 patients with moderately severe nonmalignant hypertension attending the Glasgow (Scotland) Blood Pressure Clinic were assessed. Underlying causes of hypertension were found in 297 patients (7.9%). Eighty-seven patients (2.3%) were considered to have a potentially reversible cause for their hypertension, including the oral contraceptive pill (38 patients), renovascular disease (27 patients), and primary hyperaldosteronism (ten patients), but of these only 33 patients (0.9% of total clinic population) were cured by specific intervention. Two hundred ten patients (5.6%) had irreversible renal parenchymal disease and significantly higher mortality than men and women with other causes of hypertension. Excess deaths in the renal group were attributed to renal failure (International Classification of Diseases [ICD] 580 to 589) and vascular causes (ICD 390 to 458) but not to cancer (ICD 140 to 208; 235 to 239) or other nonvascular disease. These results suggest that investigation of hypertension for an underlying cause will reveal a small number of patients with treatable disorders, of whom only a few will be cured by specific intervention, and a moderate number with irreversible disease who are at high risk of myocardial infarction and stroke.