OBJECTIVE: The objective of this investigation was to examine the relationship between BMI and waist circumference (WC) by gender and race subgroups from U.S. population-based data from 1959 to 2004 and to investigate the trend in WC over calendar time. RESEARCH METHODS AND PROCEDURES: Demographic and anthropometric cross-sectional data on 30,730 participants 18 to 79 years old across five national surveys were included. We regressed WC on BMI while controlling for age in each time period for blacks and whites by gender. RESULTS: The relationship between BMI and WC as characterized by the slope of the linear regression of WC on BMI does not seem to be changing significantly over time. A small (range, 0.08 to 0.27 cm/yr) increase in WC over time was observed. DISCUSSION: The implications of these findings for public health and for understanding any extant changes in the BMI-mortality rate relationship remain to be elucidated.