Chronic lymphocytic leukemia (CLL) is rare, but the proportion of T-CLL ishigh in Japan

Chronic lymphocytic leukemia (CLL) is a rare disease in Japan. Recent advan ces in molecular biology, diagnostic criteria and classification of CLL hav e reinforced the concept of each category of CLL as a distinct entity. Sinc e there have been no recent studies on the incidence and prevalence of CLL in Japan, the Kyushu Hematology Organization for Treatment (K-HOT) Study Gr oup conducted two studies of CLL. One study is a prospective registration o f newly diagnosed hematological disorders, which gave us some idea of the i ncidence of CLL in our region (Kyushu island) where adult T-cell leukemia i s endemic. A total of 677 patients with hematological disorders were regist ered over a 6-month period and 11 patients were diagnosed as having CLL amo ng 182 leukemia patients. This amounts to 6% of all leukemias, which is twi ce as frequent as previously reported in Japan. The other study is a retros pective analysis of CLL. Eleven institutions of the K-HOT Group analysed th eir diagnostic records of chronic lymphoid leukemia, and 145 patients with CLL were found over a period of 3-12 yr. After the data were reviewed 11 pa tients were excluded through having a different type of leukemia. The propo rtion of chronic B-cell lymphoid leukemia was 73% (98/134), while that of T -cell leukemia was 18% (24/134). The proportion of T-cell chronic leukemia was 5-6 times higher than that in Western countries. Two institutions had a complete database on hematological disorders. From this database, the annu al incidence of CLL was estimated to be 0.48 per 100 000. Thus. the inciden ce of CLL in Japan is at least 4-5 times lower than that in Western countri es, suggesting that chronic B-cell leukemia is really rare, but chronic leu kemia of T-cell lineage develops in Japan as frequently as in Western socie ties. Further investigation is required to delineate why the incidence of B -CLL is so low in Japan.