Repeated complete measurements of the red cells have been made in seven cases of familial hemolytic anemia studied in Baylor University Hospital during the past two years. The results permit some definite conclusions with regard to the diagnostic value of the size and shape of the erythrocytes (spherocytosis). Furthermore, changes in these dimensions during the course of the disease have been studied in relation to clinical variations, onset of crisis, erythropoiesis and splenectomy. Interpretation of the modifications of corpuscular size and form is based on determination of the normal range of these qualities and consideration of pathologic factors known to affect them. Alteration of the dimensions of the red cells in familial hemolytic anemia has been recognized almost as long as the disease entity itself. Chauffard1in 1907 first noted the decreased diameter, subsequently regarded as the chief characteristic by most writers, including Naegli,2Dawson3and Haden.