1. This study sought to assess the incidence of primary aldosteronism in 199 hypertensives who were normokalaemic and in whom the question of primary aldosteronism had never been raised. 2. The screening test applied was the aldosterone to renin ratio in plasma, which was raised in 40 and normal in 159 patients. A second ratio was normal in 14 of these 40. 3. Twenty-two patients with two further raised ratios required fludrocortisone suppression testing. This has been completed in 17, and failure to suppress led to a diagnosis of primary aldosteronism in all. 4. A dexamethasone suppression test (DST) excluded ACTH-dependent hyperaldosteronism and laterality of aldosterone production was determined by adrenal vein sampling. 5. Unilaterality in five patients led to adrenalectomy in four and spironolactone in one. Bilaterality in six patients led to spironolactone. 6. This study so far provides a proven (minimum) incidence for primary aldosteronism of 8.5%, a probable incidence of 12.0% (including two raised ratios) and a possible (maximum) incidence of 13.0% (leaving out those with second ratio normal). Exclusion of hypokalaemic hypertensives will lead to an underestimation of the true incidence of primary aldosteronism. 7. Based on this and other evidence, it is estimated that the incidence of primary aldosteronism in the 'essential hypertensive' population is between 5 and 15%, and is probably around 10%.