The burden of obesity is increasing in the US. Waist to thigh ratio has been proposed as a measure of central obesity. Waist thigh ratio (WTR) has been proposed as an index for abdominal (central) obesity. Epidemiological studies have demonstrated that an increased WTR is a strong predictor for type 2 diabetes and ischemic heart disease. WTR can be used as an inexpensive screening tool to detect unhealthy body shapes and to provide these individuals with the appropriate management to decrease their risk for heart disease and diabetes. Changes in waist to thigh ratio over time in representative samples of US adults were not examined before. Our objective was to examine temporal trends in waist thigh ratio among US adults. Analysis of data collected from 30,001 adults (>=20 years old) who participated in the National Health and Nutrition Examination Survey from 1988 to 1994 and continuous National Health and Nutrition Examination Surveys conducted between 1999 and 2006. The crude and age standardized waist to thigh ratio means increased among males, females and different age groups between 1988 and 2006. Age, gender, race and time were statistically significant predictors for waist thigh ratio (P values<0.01). These results document the increase in abdominal obesity among U.S. adults between 1988 and 2006. Efforts should be made to slow the increasing waist thigh ratio among in the US population.