A large number of studies have explored the relation between body mass index (BMI) and mortality in adults. The relation between BMI in adolescence and mortality has been investigated to a lesser extent. It has been suggested that all-cause mortality is elevated among those who were overweight during adolescence, but the limitation of previous studies has been study size. The present study explored this relation in a Norwegian cohort of 227,003 boys and girls, aged 14-19 years, whose height and weight were measured during tuberculosis screening in 1963-1975. These persons were followed for an average of 31.5 years (about 7.2 million person-years). A total of 7,516 deaths were registered. Multivariate Cox proportional hazards regression models were used in the analyses. An increasing risk of death by increasing BMI in adolescence was observed. Mortality among males whose baseline BMI was between the 85th and 95th percentiles and above the 95th percentile in the US reference population was 30% and 80% higher, respectively, than that among those whose baseline BMI was between the 25th and 75th percentiles. The corresponding rates among females were 30% and 100%. The excess mortality among adolescents whose BMI was high was not clearly manifested before they reached their thirties. Hence, BMI in adolescence is predictive of adult mortality.