Seven hematologists and two hematology residents independently reviewed a coded, randomized series of 62 peripheral blood films and, in each case, each decided whether or not there was morphologic evidence of iron deficiency anemia. A total of 756 independent interpretations was thus obtained in this study. Interpretations of iron deficiency anemia were made in 5.8% of the reviews of blood films obtained from 24 normal control individuals and in 49.0% of the reviews of blood films from 38 patients with proven iron deficiency anemia. Four of the participants examined the same slides twice in different random series and, on the average, contradicted 22% of their own interpretations. Except when morphologic changes are pronounced, the diagnosis of iron deficiency anemia from examination of the peripheral blood film is difficult and not very reliable.