Relationships between hemoglobin concentrations and birth outcomes have not been well characterized in African-American adolescents despite the fact that this group is at a higher risk of early childbearing. To address this issue, we characterized the prevalence of anemia and maternal factors associated with anemia in pregnant African-American adolescents. A retrospective medical chart review was undertaken of 918 adolescents who had received prenatal care at an inner-city maternity clinic between 1990 and 2000. Multiple log-linear regression analyses were used to address relationships between hemoglobin and adverse birth outcomes. The prevalence of anemia during the third trimester averaged 57-66% and was substantially higher than typically reported in adolescent and adult women. Multiparity, inadequate prenatal care, low prepregnancy BMI, history of self-reported cigarette use and infection with sexually transmitted diseases were significantly associated with lower hemoglobin during pregnancy. Adolescents with pre-eclampsia had higher hemoglobin (P < 0.01). Compared with the reference group (106-120 g/L), high hemoglobin (>120 g/L) during the second and third trimester significantly increased the risk of low birth weight (risk ratio (RR) = 3.11; [CI] 1.35, 7.13), and in the second-trimester cohort only, high hemoglobin concentrations increased the risk of preterm delivery (RR = 2.33; [CI] 1.07, 5.05). A U-shaped distribution between hemoglobin concentration and adverse birth outcomes was found in the third-trimester cohort when the reference range was decreased to 96-105 g/L to adjust for potentially lower hemoglobin concentrations among the African-American population. Our results suggest that additional medical attention may be warranted in pregnant African-American adolescents with hemoglobin concentrations of <or=95 g/L or >120 g/L.