OBJECTIVE Determine the prevalence of overweight and obesity in the Royal Thai Army (RTA) personnel. METHODS A cross-sectional study was performed on 4,276 RTA personnel aged 20-60 years between January and July 2005. They were interviewed for general health status, history of smoking, alcohol consumption, and physical activity. Blood pressure, weight, height, waist, and hip circumference were measured. RESULTS The prevalence of overweight and obesity in RTA personnel were 27.1% and 4.9% respectively. The mean value of age, weight, body mass index, waist, and hip circumference, waist hip ratio, blood pressure in overweight, and obese personnel were significantly higher than those of non-overweight personnel. Logistic regression analysis showed that significant risks of being overweight were positively associated with age, male, and working in a combat unit. Current smoking was inversely associated with overweight and obesity, whereas physical activity > or = 3 times/wk was inversely associated with obesity. Overweight and obesity were also associated with hypertension. CONCLUSIONS These baseline data are useful for future trends monitoring and may be used for health promotion planning to prevent obesity in RTA personnel.