Epidemiological studies of HTLV-I infection have demonstrated the presence of this virus in certain Amerindian populations in Central and South America. We have recently reported the first evidence of endemic HTLV-I infection in North American Amerindians from the coastal regions of British Columbia, Canada. While the predominant HTLV-I-associated disease observed in British Columbia Amerindians is the HTLV-I associated neurological disease (HTLV-I-associated myelopathy/tropical spastic paraparesis), we report here the first two cases of HTLV-I-associated adult T cell leukemia/lymphoma (ATL). Clinical and PCR evidence to support the diagnosis of HTLV-I-associated ATL in these two Amerindians is presented. Both cases of ATL were found in the same tribe although neither patient was directly related to each other. While reports of HTLV-I-associated ATL have been reported in Circumartic native peoples, reports of ATL in North American single ancestry Amerindians have not been previously made to our knowledge.