This study examined the secular trends in socioeconomic inequality in obesity during the period 1971-1994 in the United States. We analyzed the national representative data collected from three waves of the National Health and Nutrition Examination Survey (NHANES) conducted between those years. The Concentration Index was calculated to measure the socioeconomic inequality in obesity across gender, age, and ethnic groups in each survey period. In general, socioeconomic inequality in obesity was reduced between the 1970s and 1990s in women and black men, although the trend was not statistically significant for black women and was stable in white men. Our results indicate that, first, the association between obesity and socioeconomic status (SES) weakened over time, and second, SES inequality was not an important contributor to the dramatic increase in the prevalence of obesity in the United States. Our findings suggest that other social and environmental factors, which have influenced changes in people's lifestyle, might better explain the increasing overweight problem in the United States. Effective intervention efforts for the prevention and management of obesity should target all SES groups from a population perspective.