Among the patients with idiopathic pure red cell aplasia (PRCA) who do not meet the diagnostic criteria of chronic lymphocytic leukemia, there are some cases which suggest an association with clonal T lymphocytic proliferation. The morphological characteristics and responses to treatment revealed two distinct groups among the present 13 patients. The lymphocytes in one group were typical granular lymphocytes of T cell phenotype which were treated effectively with cyclophosphamide rather than cyclosporine. The response to therapy in this group occurred after a perceptible reduction in lymphocyte mass, which took at least 8 weeks. The lymphocytes in the second group consisted mainly of non-granulated lymphocytes or some granulated lymphocytes with fine and indistinct granules and responded well to cyclosporine therapy. A reduction in lymphocytes mass was not a prerequisite for the development of the remission of red cell aplasia in this group, and responses occurred within 4 weeks. Clonal T cell proliferation was detected in some patients, which raised the possibility of idiopathic PRCA being associated with a clonal proliferation of T cells. Distinguishing lymphocytes in patients with PRCA could potentially be used to plan treatment strategy and assess prognosis.