Anemia is prevalent among pregnant adolescents, but few data exist on biochemical indicators of iron status in this group. We hypothesized that among an at-risk population of African-American, pregnant adolescents, the degree of iron depletion and deficiency would be marked, and that iron deficiency anemia would comprise the majority of the observed anemia. To examine this, blood samples were collected from 80 girls (< or =18 y old) attending an inner city maternity clinic, 23 of whom were studied longitudinally in the 2nd and 3rd trimesters depending on contact at the clinic. Sample sizes for the biomarkers varied according to the blood volume available at the time the assays were completed. Descriptive statistics were applied to characterize iron status, and multivariate regression and logistic analyses were used to identify significant determinants of iron status. Depleted iron stores (ferritin < or = 15 microg/L) were indicated for 25% (n = 44) and 61% (n = 59) of adolescents during the 2nd and 3rd trimesters, respectively. Serum folate (39.3 +/- 15.4 nmol/L, n = 60), RBC folate (2378 +/- 971 nmol/L, n = 60), and serum vitamin B-12 concentrations (313 +/- 163 pmol/L, n = 60) were within normal ranges. Adolescents with serum transferrin receptor:serum ferritin ratios (R:F ratio) > 300 during the 2nd trimester were 12.5 times (95% CI 2.83, 55.25) more likely to be classified with iron deficiency anemia during the 3rd trimester (P = 0.0002) than those with lower ratios. Estimates of body iron were lower in those tested after wk 26 of gestation (P < 0.0001), and reserves were depleted in 5.0% vs. 31.3% of the 2nd (n = 40) and 3rd (n = 48) trimester cohorts, respectively. In conclusion, iron-deficiency anemia was prevalent among these pregnant minority adolescents. Targeted screening and interventions to improve diet and compliance with prenatal iron supplementation are warranted for this at-risk group.