The prevalence of primary and secondary hypertension was determined in a random sample of 7455 Swedish men aged 47 to 54 years. Three hundred and sizty-one men were undergoing treatment for hypertension. Seven hundred and ninety-eight men who had blood pressures above 175/115 mm Hg at preliminary screening were recalled for further blood pressure measurements. Those on treatment and all the untreated men whose blood pressures were still over 175/115 mm Hg then underwent extensive investigation for secondary hypertension. Renal parenchymal hypertension was found in 25 (3-6%) patients, renovascular hypertension in four (0-6%), and other forms of secondary hypertension in 11 (1-6%). The investigation led to surgical treatment in only two cases (0-3%). The low prevalence of secondary hypertension, especially surgically curable forms of hypertension, makes routine screening for these cases unnecessary, at least when patients with hypertension have been found at screening. These data must be taken into account in planning community control programmes in hypertension.