The prevalence of isolated ACTH deficiency (IAD) in Japan remains to be determined. Both authors (T.Y. & K.K.) had unique opportunities to see all patients with maturity-onset IAD in particular areas. T.Y. regularly visited Tokunoshima Island in Kagoshima Prefecture (population of about 28,500) which has two acute-care hospitals. He has taken care of endocrine-metabolism cases in one hospital and kept in touch with an endocrine-oriented physician in another. K.K. has provided glucocorticoid supplementation for all patients with maturity-onset IAD as the patients' own physician in a hospital which provided medical care for the Chuetsu district in Niigata Prefecture with 527,407 inhabitants in 2005. Four male patients (average age at onset, 70.0 years; range, 67-75 years) were identified in Tokunoshima over the 10-year period and 20 patients (15 males and 5 females; average age at onset, 63.9 years; range, 49-77 years) were cared for in the Chuetsu district in 2005. The estimated prevalence of IAD from the numbers of IAD patients and of inhabitants in the periods cited from the national population survey was 7.3 per 100,000 (an average in 10-year period) in Tokunoshima and 3.8 per 100,000 in the Chuetsu district in 2005. Maturity-onset IAD in Japan thus is not very rare in the elderly.