The National Health Service Central Register at Southport receives notifications of changes of a patient's Family Practitioner Committee. Such a change, or transfer, occurs when a patient registers with a new NHS doctor in an FPC other than that with which the patient was formerly registered. A 10% systematic sample of these transfers is taken. In Scotland, re-registrations between Area Health Boards are notified to the National Health Service Central Register, Scotland. In Northern Ireland re-registrations from other parts of the United Kingdom are notified to Central Services Agency, Belfast. (No information is available on movements within Northern Ireland). These data provide an indication of internal migration flows within the United Kingdom.