A nationwide epidemiologic survey of idiopathic hypoparathyroidism and pseudohypoparathyroidism was conducted in 1998 to clarify the prevalence of the two disorders in Japan. From a total of 14,100 departments of pediatrics, internal medicine, neurology, and endocrinology in whole Japan, 2952 (20.9%) study departments were selected at random. Of these departments receiving the first questionnaire, 1855 (62.8%) responded. From these departments 390 patients with idiopathic hypoparathyroidism and 203 with pseudohypoparathyroidism who visited the hospitals in 1997 were reported. The total numbers of patients were estimated to be 900 (690-1100) for idiopathic hypoparathyroidism and 430 (330-520) for pseudohypoparathyroidism (95% confidence intervals in parentheses). Using these data, the period prevalence of the diseases were 7.2 (5.5-8.8) per million population in idiopathic hypoparathyroidism, and 3.4 (2.6-4.2) in pseudohypoparathyroidism (95% confidence intervals in parentheses).