The National Health Interview Survey (NHIS) is a primary source of information on the changing health of the US population over the past 4 decades. The full potential of NHIS data for analyzing long-term change, however, has rarely been exploited. Time series analysis is complicated by several factors: large numbers of data files and voluminous documentation; complexity of file structures; and changing sample designs, questionnaires, and variable-coding schemes. We describe a major data integration project that will simplify cross-temporal analysis of population health data available in the NHIS. The Integrated Health Interview Series (IHIS) is a Web-based system that provides an integrated set of data and documentation based on the NHIS public use files from 1969 to the present. The Integrated Health Interview Series enhances the value of NHIS data for researchers by allowing them to make consistent comparisons across 4 decades of dramatic changes in health status, health behavior, and healthcare.