Steroid-responsive pure red cell aplasia associated with natural killer cell lymphocytosis

A patient with pure red cell aplasia and expansion of the peripheral blood natural killer (NK) cell population is described. Despite normal absolute and differential leukocyte counts, NK cells were increased at diagnosis and at relapse. Furthermore, these cells were not morphologically recognizable on the peripheral blood smear examination. A favorable clinical response to glucocorticoid therapy was accompanied by a decrease in NK cells.