OBJECTIVES We assessed changes in body mass index (BMI) among Brazilian adolescents. METHODS In 1975, 1989, and 1997, we conducted household surveys of the weights and statures of a probabilistic sample of about 50,000 Brazilian adolescents aged 10 to 19 years. Weighted prevalences were calculated and an analysis was performed with the sample design taken into account. RESULTS Adolescents of rich (southeast) and poor (northeast) regions showed a substantial increase in BMI. In the southeast, the prevalence of overweight, defined by international age- and gender-specific BMI cutoffs, for both genders reached 17% in 1997, whereas in the northeast, the prevalence tripled, reaching 5% among boys and 12% among girls. Older girls living in urban areas in the southeast showed a decrease in prevalence from 16% to 13% in the latter 2 surveys. For all boys and for young girls, the BMI values for the 85th percentile in 1997 were much higher than the 95th percentile values in 1975. CONCLUSIONS BMI increased dramatically in Brazilian adolescents, mainly among boys; among older girls from the richest region, the prevalence of overweight is decreasing.