- The identification of young adult populations at risk for overweight and obesity is important for the formulation of intervention programs. We assessed the risk factors associated with overweight and obesity and increased body mass index (BMI) in a population of healthy young adults during military service. We analyzed data from an ongoing survey of a representative sample of 11,391 males and 11,280 females aged 20-22 years, interviewed and examined upon release from military service in Israel between 1989 and 2003. Increase in BMI as well as incidence of overweight (BMI <25-<30 kg/m2) and of obesity (BMI≥30 kg/m2) during military service were measured. The average increase in BMI during military service was 1.11 kg/m2 in males and 1.08 kg/m2 in females. A greater increase was positively associated with low paternal education and smoking cessation, and negatively associated with a high level of physical activity. Twelve percent of participants with a normal BMI on recruitment became overweight, and 21.7% of overweight subjects became obese. A higher incidence of overweight was associated with a low level of education (subject and father) in both sexes, as well as nonuse of oral contraceptives and a low level of physical activity among females. In summary, there appears to be a significant increase in BMI during early adulthood. The population at higher risk for becoming obese is characterized by low paternal education, smoking cessation, and a low level of physical activity. These higher risk groups should be targeted for intervention programs.