The transition from adolescence to adulthood is associated with increased risk for excess weight gain. Transitioning adults are relatively difficult to study, however, because unless they matriculate to secondary education there are few opportunities to track them. Thus, demographic and socioeconomic factors associated with overweight obesity are not well-documented, much less so among adolescents who have maintained normal weight throughout adolescence. We examined the incidence of overweight and obesity among 10,723 healthy, normal weight (body mass index [BMI] < 25 kg/m
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)18-year olds in 2008 who remained members of Kaiser Permanente Southern California through 2012. Sex, race/ethnicity, height, and weight were recorded in electronic medical records; BMI was calculated. Neighborhood education was determined from US 2000 Census block group data, and quartiles defined by the proportion of adults with an education below high school. We followed 3,677 men (BMI: 21.5±1.96) and 7,046 women (BMI: 21.2±2.0). Relative risks (RR) and 95% confidence intervals (CI) were determined controlling for sex, race/ethnicity, neighborhood education, and smoking history. During 34,660 person-years of follow-up (mean 3.23±1.34 years), we documented 2,672 incident cases of overweight (24.9%) and 237 of obesity (2.2%). The RR of overweight was 1.25 (95% CI: 1.13, 1.38) in the lowest quartile of neighborhood education compared with the highest quartile (test for trend p=0.001). The RR for obesity was 1.50 (95% CI: 1.00, 2.25; test for trend p=0.025). While women were less likely to become overweight than men (RR 0.88, 95% CI: 0.82, 0.94), they were twice as likely to become obese (RR: 1.99, 95% CI: 1.45, 2.72). Hispanics had greater risk of overweight (RR: 1.09, 95% CI: 1.00, 1.18) and Blacks had greater risk of obesity (RR: 1.46, 95% CI: 1.00, 2.15) than non-Hispanic whites. Compared with their non-Hispanic white peers, Asians had lower risk of overweight and obesity (RR: 0.79, 95% CI: 0.67, 0.93 and RR: 0.46, 95% CI: 0.21, 0.99, respectively). In conclusion, normal weight adolescents are at risk for becoming overweight as they transition to adulthood. Sociodemographic factors predict incidence of overweight and obesity. Women and Blacks are at particular risk for excessive weight gain trajectories.