OBJECTIVES: To estimate prevalence trends of overweight and obesity and to assess relationship between weight status and physical activity in adults ³ 20 years old. METHODS: Three National Health and Nutrition Examination Surveys, NHANES (II) for 1976 - 1980, NHANES (III) for 1988 - 1994, and NHANES (IV) 1999 - 2002 were used. Based on body mass index (BMI), adults were classified as overweight (BMI 25 to < 30) or obese (BMI ³ 30). Duration of physical activity was calculated using leisure-time physical activity to determine compliance of CDC recommendations. SAS and SUDAAN softwares were used to account for the complex survey designs. RESULTS: The mean BMI increased from 25.31(II) to 28.02(IV). During the study period of II-IV, the prevalence of overweight and obesity changed from 31.4% to 34.7% and from 14.5% to 30.2%, respectively. Obesity significantly increased 7.8% for II-III (p <0.001) and 7.9% for III-IV (p <0.001). The prevalence of overweight individuals increased most rapidly in adults 20-39 years old. Obesity increased most rapidly in patients 40-64 years old for II-III and 20-39 years old for III-IV. Hypertension was more prevalent in obesity (75.2%(II), 50.7%(III), 48.0%(IV)) than overweight (54.5%(II), 33.7%(III), 34.1%(IV)), yet for both groups decreased over time. During IV, overweight adults spent an average of 240 minutes per week for physical activity, and obese adults spent 170 minutes. Only 43.2% of overweight and 37.5% of obese took medications or followed physical activity guideline to control weight. CONCLUSIONS: Two-thirds of U.S. adults were considered to be overweight or obese. The prevalence of overweight and obesity continuously increased during the past 30 years and increased more rapidly in recent years. It is recommended that aggressive interventions be initiated to increase patient awareness so that more patients get treated and follow physical activity guidelines to lose or control weight.