Abnormalities in serum calcium level usually lead to the diagnosis of parathyroid diseases. Therefore, parathyroid diseases can be classified according to serum calcium levels of the patients. However, the present classification of the diseases have several problems. First, the existence of some of the historically wellknown parathyroid diseases is still under controversy at present. Second, recent studies using molecular and cellular biological approaches have uncovered several genetic abnormalities in some parathyroid tumors, suggesting that parathyroid adenoma or cancer is not a homogeneous disease entity. Third, there remain many unanswered problems to explain the pathogenesis of various parathyroid diseases including pseudohypoparathyroidism. Further elucidation of the pathogenesis of parathyroid diseases should enable us to establish a better classification of these diseases.