Abstract Study Objective To evaluate whether ovulatory dysfunction due to polycystic ovary syndrome (PCOS) is a common underlying etiology of abnormal uterine bleeding (AUB) in adolescents requiring hospitalization and to explore etiology, treatment and complications of AUB with severe anemia in adolescents. Design We identified females aged 8 to 20 years admitted to a children’s hospital for treatment of AUB from January 2000 to December 2014. Our hospital protocol advises hormonal testing for PCOS and other disorders prior to treatment for AUB. We reviewed medical records and recorded laboratory evaluations, treatments and final underlying diagnoses as well as recurrences of AUB and readmissions in the subsequent year. Results Of the 125 subjects, mean age was 16.5 ± 2.9 years; mean hemoglobin level was 7.0 ± 1.8 g/dL; 54% overweight/obese; 41% sexually active. PCOS accounted for 33% of admissions; hypothalamic pituitary ovarian (HPO) axis immaturity 31%; endometritis 13%; bleeding disorders 10%. Girls with PCOS were more likely to be overweight/obese 74% vs. 46%, p<0.01 and girls with HPO axis immaturity had lower hemoglobin levels 6.4g/dL vs. 7.4g/dL, p<0.05, than girls with all other etiologies of AUB. Treating physicians failed to diagnose endometritis as the etiology for AUB in 4/8 girls with positive tests for sexually transmitted infection and no other etiology. Conclusions PCOS was the most common underlying etiology in adolescents hospitalized with AUB. Screening for hyperandrogenemia is important for early diagnosis of PCOS to allow ongoing management and prevention of comorbidities. Endometritis was frequently underestimated as an etiology for AUB.