Adult T-cell leukemia/lymphoma (ATLL) is a peripheral T cell neoplasm that is associated with infection by the human T-cell leukemia virus type I (HTLV-1). Although the high incidence of ATLL in HTLV-1-endemic areas is well known, population-based evidence concerning the incidence of ATLL in non-endemic areas is scarce. To answer this, we estimated the age-standardized incidence of ATLL from 1993 to 2006 for Japan and 1993 to 2008 for the US and assessed its trend using data from a population-based cancer registry in Japan and Surveillance Epidemiology and End Results (SEER) in the US. The Japanese data were collected from 15 prefectures. A total of 2055 patients in the three prefectures in Kyushu and 1380 patients in the 12 prefectures in Honshu were diagnosed with ATLL in the study period. In the US, a total of 140 patients were diagnosed with ATLL. The results showed that the age-standardized incidence in non-endemic areas in Japan and in the US significantly increased during this period (annual percent change [95%CI]; Japan-Honshu: +4.6% [1.1, 8.2]; US: +6.2% [1.5, 11.1]), while in the endemic areas of Japan there was no change (annual percent change [95%CI]; Japan-Kyushu: 0.0% [−1.6, 1.7]). This result indicates that the disease has been spread by carriers to non-endemic areas, and suggests the necessity of establishing a standard preventive strategy.