We reviewed the records at the Mayo Clinic for the years 1973, 1974, and 1975 to determine the number of patients with hypertension who had had operations for repair of renal artery stenosis, excision of pheochromocytoma, or resection of aldosterone-producing adenoma. During the years studied, the average numbers of procedures per year were, respectively, 46.7, 10.3, and 2.7. For the purpose of estimating the frequency of each one of these three conditions among the population of hypertensive patients examined at the Mayo Clinic, we applied age- and sex-specific incidence figures from the US National Health Survey to the 162-273 patients examined who were more than 15 years old in 1974. We estimate that there were 26,589 patients who had diastolic blood pressures equal to or greater than 95 mm Hg. The indices generated estimated that renal artery stenosis repair was done in 18/10,000 (0.18%) hypertensive patients, pheochromocytoma excision in 4/10,000 (0.04%), and aldosterone-producing adenoma resection in 1/108000 (0.01%). These indices are strikingly lower than those frequently reported elsewhere, suggesting that these conditions are truly rare among hypertensive patients seen in clinical practice.