The National Longitudinal Study of the High School Class of 1972 (NLS) critical data base contains 151 items (plus background information) from the base year and followup questionnaires; about thirty-seven percent of all items. This set of critical items consists of: (1) basic demographic variables; (2) items necessary for defining activity states (occupations); (3) key items from the first and second followups; (4) base year items included in the resurvey section of the first followup questionnaire; and (5) critical items based on rankings by the National Center for Education Statistics (NCES) and a users committee formed by NCES to provide guidance in questionnaire development. The items in the critical data base, particularly those for the followup questionnaires, may be used as a check list of crucial questions to be included in future followups. Appended tables list the item numbers for each survey included in the critical data base and provide item descriptions. The relationship of the planned third followup survey to the critical data base is discussed. (Author/CP)