A collaborative study of nutritional anemia in third trimester pregnancy was performed in seven Latin American countries. Laboratory measurements included hemoglobin level, mean corpuscular hemoglobin concentration (MCHC), serum iron and iron-binding capacity, serum folate, vitamin B12 and albumin. Iron deficiency (transferrin saturation below 15%) was found in 48% of pregnant women, as compared with 21% of nonpregnant females and 3% of male controls of comparable age. The prevalence of folate deficiency (serum folate below 3 ng/ml.) was 10%, 10% and 9% in these three groups, respectively. Vitamin B12 deficiency (serum level below 80 pg/ml.) was found in 15% of pregnant women, but in less than 1% of both control groups. Anemia, as defined by current WHO criteria, was found in 38.5% of pregnant women, 17.3% of nonpregnant women and 3.9% of men. Analysis of the frequency distribution for hemoglobin levels, based on a Gaussian distribution in normal subjects, suggested that a large portion of subjects considered anemic by WHO criteria were normal and that the true incidence of anemia in pregnant and nonpregnant females was 22 and 12% respectively. Correlation analysis indicated that iron deficiency was of major importance as a cause of anemia, while folate lack was contributory only in pregnancy; no relationship could be demonstrated between vitamin B12 deficiency and anemia.