Summary Splenic pooling of red cells and an expanded plasma volume are considered to be among the major mechanisms responsible for the anaemia in hypersplenism. In those conditions in which massive splenomegaly is associated with various degrees of marrow failure, diagnosis of the cause of anaemia may be difficult. A simple technique was used to estimate the degree of hypersplenism, from red cell mass data, in 94 patients with unequivocal lymphoproliferative or myeloproliferative disorders. The splenic effect was found to correlate well with both the size of the spleen (r = 0.75–0.90) and the actual red cell mass (0.79), and was abolished by splenectomy. Clinical data is also presented on 43 of these patients who underwent splenectomy. The incidence and type of complications, survival figures, and possible criteria for patient selection are discussed.