The Therapeutic Intervention Scoring System (TISS) introduced in 1974 has become a widely accepted method of classifying critically ill patients. In response to requests to update the system because of recent innovations in critical care, some items have been deleted, some have been added, and certain point scores have been adjusted. Explanations of items within the system and guidelines for the user are included. A comparison of the new 1983 system to the old 1974 system in 100 consecutive patients reveals no difference in total point scores. We hope this updated explanation will ease the task of assessing use of intensive care services.