On the basis of the response to EDTA induced hypocalcemia and the pretreatment plasma calcium level, 5 grades of severity of hypoparathyroidism are denned. The plasma levels (mean ± SE in mg/100 ml) of calcium (Ca) and phosphorus (P) in 62 patients with surgical hypoparathyroidism (SHP) from a single hospital were: Grade 1-Ca 9.29 ± 0.09, P 3.49 ± 0.11 (n = 11); Grade 2-Ca 8.80 ± 0.06, P 3.58 ± 0.24 (n = 10); Grade 3-Ca 8.06 ± 0.08, P 3.85 ± 0.17 (n = 14); Grade 4-Ca 6.99 ± 0.06, P 4.96 ± 0.18 (n = 22); Grade 5-Ca 5.62 ± 0.16, P 5.97 ± 0.23 (n = 16). Nine patients were included in two or more grades on different occasions. There was a significant inverse correlation between Ca and P levels (r = 0.748, p < 0.001). Comparison of the results with those from other series of SHP, idiopathic hypoparathyroidism (IHP) and pseudohypoparathyroidism (PHP) suggests the following conclusions: 1. There is a clear demarcation of abnormal from normal plasma Ca levels in IHP, but not in SHP or PHP. 2. IHP is on the average a more severe disorder than PHP which in turn is more severe than SHP. 3. The plasma P level falls with increasing age in IHP and in normal subjects but not in PHP. 4. Plasma P levels in adults are the same in IHP and SHP for the same degree of hypocalcemia, but are higher in PHP than in either SHP or IHP. 5. Mild (grade 3) cases of SHP are characterized by hypocalcemia, normophosphatemia and a high rate of spontaneous improvement with time, whereas mild cases of PHP may have normocalcemia and hyperphosphatemia. 6. The plasma P in IHP, PHP and SHP is higher in the United States than elsewhere. 7. A fall in parathyroid hormone secretion has a progressively greater effect on the plasma P level as zero secretion is approached. 8. Most patients with SHP retain some functioning parathyroid tissue even after many years. 9. Failure of this tissue remnant to regenerate may be partly due to permanent parathyroid ischemia as a result of thyroid surgery.