Objective: To test the hypothesis that adolescent body mass index (BMI) tracks into adulthood and can be used as a predictor of obesity and/or central adiposity in adulthood. Method: A prospective cohort study following up 111 female and 84 male subjects who participated in dietary and anthropometric surveys when aged 12 years (in 1979–1981) and 33 years (in 2000–2001). At both time points, height and weight were measured and BMI calculated. At 33 years, waist circumference (WC) and hip circumference were also measured and waist-to-hip ratio (WHR) calculated. Results: In the male and female participants, BMI at 12 years was associated significantly with BMI at 33 years (R = 0.58 and 0.53, respectively, both p < 0.01) and WC at 33 years (R = 0.58 and 0.53, both p < 0.01). The probability of being an obese adult increased with rising adolescent BMI: normal weight male (BMI < 20.89 kg/m2) and female subjects (BMI < 21.20 kg/m2) at 12 years had a 20% and a 7% chance of being obese at 33 years, respectively; the probabilities for obese male (BMI ≥25.58kg/m2) and female subjects (BMI ≥26.05kg/m2) were 83 and 64%. The corresponding probability of becoming centrally obese (measured by WC) increased from 17 and 16% in male and female subjects of a normal weight to 58 and 59% in those being obese. Conclusions: Adolescent BMI is a good predictor of adult BMI and WC and the likelihood of becoming obese and/or centrally obese in adulthood.