- In view of the rising prevalence of obesity, the identification of young adult populations at risk is important for the formulation of intervention and prevention programs. To assess demographic and behavioral factors associated with an increase in body mass index in young healthy adults and to identify the incidence of overweight/obesity in this population. Data on anthropometric measures, demographic characteristics, and health behaviors were collected retrospectively for a representative sample of young Israeli adults (11,391 men, 11,280 women) on their release from military service (age 20-22 years) between 1989 and 2003. The incidence of overweight (BMI < 25 < or =30 kg/m2), incidence of obesity (BMI > or =30 kg/m2), and increase in BMI during military service were calculated. The average increase in BMI during military service was 1.11 kg/m2 in males and 1.08 kg/m2 in females. Agreater increase was positively associated with low paternal education and smoking cessation, and negatively associated with high physical activity. Twelve percent of subjects with a normal BMI on recruitment became overweight, and 21.7% of overweight subjects became obese. On multivariate logistic regression analysis, a higher incidence of overweight was associated with low education level (in both the subject and his or her father) in both genders, and non-use of oral contraceptives and low level of physical activity in females. BMI appears to increase significantly during early adulthood. Intervention programs should be targeted specifically at subjects with low education or who started smoking before age 18, and physical activity (especially among females) should be encouraged.