1. Of 93 patients with primary aldosteronism seen during a 20 year period, 52 had an aldosterone-producing adenoma (APA) removed (five more await surgery), 14 had bilateral adrenal hyperplasia (BAH), three had glucocorticoid-suppressible hyperaldosteronism (GSH), one had adrenal carcinoma and 18 are yet to be categorized. 2. Seventy-three presented with hypertension and hypokalaemia. Others had markedly suppressed renal venous plasma renin activity (PRA) or elevated plasma aldosterone (PA)/PRA ratio, in new or resistant hypertensives. 3. The PA/PRA ratio was the most reliable screening test. 4. Diagnosis depended on the failure of suppression of aldosterone by salt loading and fludrocortisone. 5. Differentiation of BAH from APA depended on adrenal venous sampling comparing adrenal and peripheral venous PA/cortisol ratios. 6. A new familial variety of primary aldosteronism is described, with two affected members in each of three families. 7. Primary aldosteronism should be looked for in resistant and low-renin hypertension as well as in hypertension with hypokalaemia, and other family members should have PA/PRA measured if they are hypertensive.