The haematological results of 200 patients with uncomplicated thyrotoxicosis is reviewed. Although more than half of the patients showed a normal blood picture, several abnormalities were detected. In 37 percent of patients the erythrocytes showed microcytosis, and 8.5 percent were mildly anaemic. An absolute lymphocytosis was found in 11 percent and neutropenia was present in 2.5 percent of patients. The prevalence of treated pernicious anaemia was 1.5 percent.