This is the first study to establish the prevalence of overweight and obesity among Ecuadorian adolescent students. The population studied was made up of 2.829 students, 1.461 females and 1.368 males between 12 and <19 years of age. One thousand four hundred and thirty five students were registered in 60 schools, public and private, in the six main cities of the Coastal Region; the remaining 1.394 students were registered in 60 schools, public and private, in the six main cities in the Andean Region. Height and weight were measured in all participants and the body mass index (BMI) of each individual was calculated. Overweight was diagnosed in those adolescents whose BMI was between percentiles 85 and < 95, and obesity was diagnosed in the subjects whose BMI was > 95. Results indicate that 21.2% of adolescents had excess weight: 13.7% were overweight and 7.5% had obesity. Excess weight was higher in the Coast (24.7%) than in the Andean Region (17.7%; P < 0.0001). In the same way, excess weight was higher among students attending private schools (25,3%) than in those attending public schools (18.9%; P < 0.0001). Data also indicate that excess weight was more common in women than in men, 21.5% versus 20.8%, respectively (P < 0.02). The study also indicated that 16.8% of adolescents were underweight. Taken together, these data indicate that 38% of the studied population was malnourished. It is necessary to take measures to prevent and treat these important public health problems in Ecuador.