Secular trends in the relationship of body mass index (BMI) and skinfold thickness to educational and income levels were examined for white and black women aged 18-34 y over the period 1960-80 with data from three successive national surveys: NHES Cycle I, NHANES I, and NHANES II. Statistical models were fitted to assess the variability in mean BMI over time within levels of education or income. Mean BMI and skinfold thickness were negatively associated with both education and income. Over the 20-y period mean BMI increased for both white and black women at all levels of income and education. Because the increase in mean BMI was greater at lower educational levels, the differentiation of BMI by educational level increased over time, yielding a stronger negative association. In contrast differentiation of BMI by income category decreased over time. Black-white differences in mean BMI at comparable educational and income levels persisted over this period.