The prevalence, clinical characteristics, and prognostic significance of immune thrombocytopenia (IT) in patients with chronic lymphocytic leukemia (CLL) have not been clearly determined. To clarify this, we retrospectively analyzed 1278 consecutive newly diagnosed patients with CLL. Criteria for IT diagnosis included the following: rapid (< 2 weeks) and severe fall (half of the initial level and below 100 x 10(9)/L) in platelet count; normal or augmented megakaryocytes in bone marrow; no or limited (not palpable) splenomegaly; no cytotoxic treatment in the preceding month. Sixty-four patients (5%) were diagnosed with IT. The median time to IT from CLL diagnosis was 13 months (range, 0-81 months), and median platelet count at IT diagnosis was 14 x 10(9)/L (range, 1-71 x 10(9)/L). Fifty-six of the 64 patients (87%) received treatment for IT. The probability of responding to treatment for IT was significantly higher for patients receiving chemotherapy with or without steroids than for patients treated with intravenous immunoglobulins with or without steroids (P = .01). The development of IT was significantly associated with unmutated IgVh, a positive direct antiglobulin test, and the occurrence of autoimmune hemolytic anemia. Patients with CLL and IT had poorer survival than other patients with CLL (5-year overall survival 64% vs 82%, P < .001), and this effect was independent from common clinical prognostic variables.