Dear Sir, Adult T-cell leukemia/lymphoma (ATL) is a malignancy of mature T cells which develops among individuals infected with human T-cell lymphotropic virus type-1 (HTLV-1). ATL shows characteristic geographic distribution in the world, being endemic in south-western Japan, the Caribbean region, central Africa, etc. Previous epidemiologic studies have shown that the incidence rate of ATL among carriers of HTLV-1 ranges from 0.6 to 2.0 cases/1,000 person-years, and the life-time risk of ATL from 1.0 to 6.6%. 1,2 However, there have been few reports on the secular trend of the incidence rate of ATL in geographically defined populations. In this study, the authors investigated the changes in the ATL incidence in Nagasaki Prefecture, Japan, using a population-based cancer registry. The study area of the present study was the whole of Nagasaki Prefecture, Japan, the total population of which included 726,894 men and 818,040 women in 1995. In this prefecture, a population-based cancer registry (Nagasaki Prefectural Cancer Registry, NPCR) has been operating since 1985, under the supervision of the Department of Epidemiology, Radiation Effects Research Foundation. The rate of incidence of ATL was calculated by dividing the number of cases by person-years of observation. The incident cases of ATL diagnosed from 1985 to 2004 were selected from the file of the NPCR. In the NPCR, patients satisfying the following criteria have been registered as ATL: (i) Tcell malignancy with seropositivity to HTLV-1 or (ii) clinical diagnosis of ATL. The validity of the diagnosis has been reported in a previous paper. The person-years at risk were calculated by summing the population of each year during the observation period. The age-standardized incidence rates of ATL and their 95% confidence intervals (CI) were calculated by the conventional methods, using the world population as a reference. Poisson regression analysis (SAS GENMOD procedure, version 8.2) and Mantel-Haenszel procedure were used to estimate the sex-adjusted or sex-and-age-adjusted incidence rate ratios (RR) and their 95% CI. The study protocol was reviewed and approved by the Ethical Committee of the Tokushima University Hospital. There were 1,851 registered cases of ATL (1,022 males and 829 females) between 1985 and 2004. The proportion of patients registered by the death certificates only was 8.9%. Figure shows the distribution of age at onset of