From 1980 through 1994, we identified 47 adult patients with acquired pure red cell aplasia (median age, 64 years; range, 22 to 84 years). Associated clinical disorders included T-cell large granular lymphocytic (LGL) leukemia, thymoma, chronic lymphocytic leukemia, and non-Hodgkin's lymphoma. Review of bone marrow findings in 40 patients showed absence of erythroid precursors in 14 patients and rare pronormoblasts in 26. None had morphologic evidence of myelodysplasia. T-cell receptor gene rearrangement studies with Southern blot technique in 14 patients showed clonal rearrangements in nine. Karyotypic analyses performed in 28 patients showed clonal abnormalities in four. Overall, 28 of 47 patients (60%) responded to immunosuppressive therapy, but none were the patients with cytogenetic abnormalities. There was a trend toward superior response to immunosuppressive agents in the patients with T-cell LGL leukemia. Cyclophosphamide, with or without corticosteroids, was the most useful treatment agent. Cyclosporine A was effective for refractory disease. Neither the presence of an associated clinical disorder nor the existence of detectable erythroid precursors affected overall survival. We conclude that (1) T-cell LGL leukemia is the disorder most commonly associated with pure red cell aplasia, (2) the presence of clonal cytogenetic abnormality predicts poor response to immunosuppressive therapy, and (3) oral cyclophosphamide and cyclosporine A are effective treatment regimens.