We estimated the prevalence of anemia in the United States from the results of the Second National Health and Nutrition Examination Survey ( NHANES II, 1976 to 1980). Reference ranges for Hb were first derived from 11,547 subjects in whom laboratory values for serum iron/iron-binding capacity, mean corpuscular volume, and erythrocyte protoporphyrin were all normal (greater than or equal to 16%, greater than or equal to 80 fl, and less than or equal to 75 micrograms/dl red blood cells, respectively). Using these reference standards, the prevalence of anemia (Hb values below the 95% reference range for age and sex) among the 15,093 subjects with complete laboratory results was highest in infants (5.7%), teenage girls (5.9%), young women (5.8%), and elderly men (4.4%). The pattern of laboratory abnormalities in anemic subjects indicated that iron deficiency predominated as a cause in infants and young women in contrast to inflammatory disease in the elderly.