Patients belonging to four families with 'atypical elliptocytosis' have been investigated. Clinical, haematological, erythrokinetic and enzymatic characteristics as well as the effect of splenectomy are discussed. These studies appear to define the fundamental features of a particular disorder or a variety of hereditary elliptocytosis; characterized by a genetic autosomal dominant character, moderate degree of RBC eccentricity, erythroid dysplasia with relative marrow failure and incomplete response to splenectomy.