A 74-year-old Japanese male was admitted because of anemia. Hepatosplenomegaly, lymphoadenopathy, and purpura were not found. The laboratory data on admission revealed that the white-cell count was 9,400/microliters, the hemoglobin 11.1 g/dl, and the platelet count 17,000/microliters. Platelet-associated IgG was 794.2 ng/10(7) cells. The patient was diagnosed as having autoimmune thrombocytopenic purpura (ATP) at this time. He was treated with prednisolone, but his thrombocytopenia not improve. In addition to prednisolone, azathioprine was given to him. During the course of treatment, leukocytosis gradually appeared and the white-cell count reached more than 30,000/microliters with over 70% lymphocytes. A bone marrow aspiration revealed 70% of small lymphocytes, and surface marker analysis showed that CD19 and HLA-DR were positive on these lymphocytes. Southern blotting analysis demonstrated rearrangements of JH and JK. He was finally diagnosed as B-CLL complicated by ATP. One month after the azathioprine administration, the platelet count increased more than 30,000/microliters and the white-cell count decreased less than 10,000/microliters. About 2% of patients with CLL are known to be complicated by ATP. To our knowledge, the present case is the first case of B-CLL complicated by ATP in Japan.