The prevalence of adolescent anemia, iron deficiency and thalassemia were examined in 2 provinces of northeast Thailand. Blood specimens were collected from adolescent subjects aged 15–17 years in 2 areas; 185 (85 males and 100 females) in Mukdahan province and 313 (116 males and 197 females) in Roi-Et. RBC parameters, serum ferritin levels, Hb and DNA analyses for the identification of common thalassemia genes in Thailand were investigated. The prevalences of anemia were found to be 21.1% (8.1 in male and 13.0 in female) and 16.6% (8.9 in male and 7.7 in female) in Mukdahan and Roi-Et province, respectively. Iron deficiency was observed to be 24.3% in Mukdahan and 14.7% in Roi-Et. Various types of thalassemia were identified in 62.2 and 58.8% of the subject populations, respectively. The proportions of iron deficiency, thalassemia and combined thalassemia and iron deficiency among anemic subjects were 10.2, 53.8 and 30.8% in Mukdahan, and 7.7, 67.3 and 9.6% in Roi-Et. Hematological characteristics were analyzed and are presented. It is concluded that thalassemia and hemoglobinopathies rather than iron deficiency are major causes of adolescent anemia which should be taken into account in public health strategies for the control of anemia in the region.