SummaryThe prevalence of Addison's disease (chronic adrenal failure) has not been widely investigated and is usually given as 39 in a million. We conducted a prevalence study using a postal survey of general practitioners in Coventry. Three quarters (1391188) replied, representing 79/85 (93%) of the practices. Thirty cases of Addision's disease were found from a total patient list of 323 852, of which a third were tuberculous in origin and twothirds non-tuberculous (12I30 autoimmune, 8/30 unclassified). We conclude that Addison's disease is 2.4 times more common than previously reported. The tuberculous group was older, 65 vs 52 years (p<0.05), and had had the disease for longer than the non-tuberculous group, 20 vs 12 years (p<0.05). There was no significant difference in the age at diagnosis.