To determine the number of patients fulfilling recently issued national guidelines on the use of ICDs in patients with arrhythmias, the authors undertook two observational audits of clinical records. The first audit included patients investigated and treated at a tertiary referral cardiothoracic center during a 1-month period, and the second included patients admitted to the three coronary care units serving a circumscribed district population during a second month. Patient records were audited against the recommendations for ICD implantation made by the National Institute for Clinical Excellence to determine if the patient fulfilled the criteria for ICD implantation. The audit was repeated with the same patient records against the MADIT 2 selection criteria. The audit identified underprovision of ICD therapy in the United Kingdom for a variety of reasons. It also demonstrated that the number of patients fulfilling selection criteria defined by the national guidelines for ICD implantation is far in excess of the numbers predicted. The annual incidence of patients fulfilling national criteria is about 150/million, with an additional "prevalence" of at least 41/million. Applying the less restrictive MADIT II criteria to select patients for ICDs as a primary prevention increased the numbers to 504/million ("new incidence") and 311/million ("prevalence") per year, in excess of the predictions by a factor of between 10 and 25.