The laboratory evaluation of anemia begins with a complete blood count and reticulocyte count. The anemia is then categorized as microcytic, macrocytic or normocytic, with or without reticulocytosis. Examination of the peripheral smear and a small number of specific tests confirm the diagnosis. The serum iron level, total iron-binding capacity, serum ferritin level and hemoglobin electrophoresis generally separate the microcytic anemias. The erythrocyte size-distribution width may be particularly helpful in distinguishing iron deficiency from thalassemia minor. Significant changes have occurred in the laboratory evaluation of macrocytic anemia, and a new syndrome of nitrous oxide-induced megaloblastosis and neurologic dysfunction has been recognized. A suggested approach to the hemolytic anemias includes using the micro-Coombs' test and ektacytometry. Finally, a number of causes have been identified for normocytic anemia without reticulocytosis, including normocytic megaloblastic anemia and the acquired immunodeficiency syndrome.