A nutrition survey was conducted in the rural Lindi District of Tanzania to determine the magnitude of anemia and iron deficiency in different age and sex groups as related to nutritional status, parasitic infections, food iron intake, and socioeconomic factors. In a 30-cluster sampling design, 660 households were randomly selected and a total of 2320 subjects aged 6 mo to 65 y were examined. Iron status was assessed by measuring hemoglobin and erythrocyte protoporphyrin in a finger-prick sample: 55% of the subjects had anemia and 61% of the anemia was associated with iron deficiency (erythrocyte protoporphyrin > 125 micromol/mol heme). Preschool children (aged < 5 y) were the most affected; 84% were anemic (hemoglobin < 110 g/L). Fifty percent of the nonanemic preschool children and approximately 90% of all the severely anemic subjects were iron deficient. Hemoglobin was lower in schoolchildren (aged 5-14 y) and in adolescent and adult males (aged > or = 15 y) with a low body mass index. Parasitic infections were only associated with anemia and iron deficiency in schoolchildren and adolescent and adult males. Malaria was associated with anemia (P < 0.001), whereas schistosomiasis was associated with anemia and iron deficiency (P < 0.001 and P < 0.05, respectively). Hookworm infestation was associated with iron deficiency (P < 0.05) and with anemia (P < 0.01) only in adolescents and adults. A mainly cereal-based diet with additional legumes and green vegetables was found by in vitro tests to contain high amounts of total iron but of low bioavailability. Estimation of the amount of iron absorbed confirmed inadequate iron nutrition. Although anemia is a result of a synergism of a variety of causes, iron deficiency remains the major cause.