Objective : This study was designed to describe the impacts in body mass index (BMI) during 6 years (2002-2008) and to identify associations between these impacts and the amount of physical activity of South Korean short-term active-duty officer and noncommissioned officer. Method : This study targeted the 44,758 men of the 49,916 career soldiers who had undergone over the two medical examinations conducted by the National Health Insurance Corporation; 3,045 soldiers with missing data on height, weight, physical activity, 1,956 female soldiers and 157 soldiers with small data on study type were excluded. A linear mixed-regression model was used to categorize changes in BMI due to age versus those due to amount of physical activity. Results : Career soldiers experienced significant increases in BMI compared with baseline data gathered in first enlisted year. The increases in each enlisted year were as follows : second enlisted year: 0.27, third enlisted year: 0.60. The differences between baseline and final BMIs were: -0.04 second enlisted year exercising 3-4 times per week, -0.13 third enlisted year exercising 3-4 times per week, -0.13 second enlisted year exercising 5-6 times per week, -27 third enlisted year exercising 5-6 times per week, -0.12 second enlisted year exercising 7 times per week, -0.23 third enlisted year exercising 7 times per week. Conclusions : Obesity in South Korean short-term active duty officers and noncommissioned officers increased markedly between first enlisted year and third enlisted year, and our data showed that the amount of physical activity was inversely related to increases in BMI. Policies to prevent obesity are needed to reduce this impact.