Objective: Anemia in adolescents is a significant health problem caused by multiple factors. With this study, we aimed to investigate anemia aetiology and its frequency among the adolescents.Material and Method: A total of 74 patients were admitted to Pediatrics Clinic of Istanbul Şişli Hamidiye Etfal Training and Research Hospital between January 2012 and August 2013, who were aged between 10-19 years and diagnosed with anemia, were investigated regarding the aetiology.Results: Female and male ratio were 74.3% and 25.7%, respectively, and the mean age of the patients was 14.7±1.8 years. The most common symptom was fatigue. Red meat consumption was detected to be low in patients who were diagnosed with iron deficiency anemia (IDA). Six of the patients had a history of bleeding. Menorrhagia was present in 23.6% of the females. As a result of certain laboratory tests, 81.1% of the patients were diagnosed with IDA. The aetiological evaluation revealed that the most common reason for the anemia was nutritional. Vitamin B12 deficiency and folic acid deficiency anemia were diagnosed in 13.5% and 1.4% of patients, respectively. Eleven patients underwent endoscopy, and the most common finding was gastritis. Apart from these diagnosis, we reported acute leukaemia in 3 patients, sepsis in 3 patients, hereditary spherocytosis, autoimmune hepatitis, autoimmune hemolytic anemia, aplastic anemia, a chronic renal failure and cholangitis in one patient each. While 39.2% of the patients were treated with hospitalization, 60.8% of the patients were under outpatient follow-up and treated.Conclusion: An inappropriate diet and eating habit is the most common aetiology of anemia among the adolescents. Considering that this age group of patients are not under routine health care followup of pediatricians, it is necessary to examine the adolescent for anemia for early diagnosis, with investigating the causes and selecting the appropriate treatment.