Six Colombian patients with adult T-cell leukemia/lymphoma (ATL) are presented. The clinical and hematological features, the familial clusters of human T lymphotropic virus type 1 (HTLV-I) carriers and the prognoses of the Colombian ATL patients were similar to those previously reported for Japanese ATL patients. The only difference was the mean age of onset, which was more than 20 years younger than in Japanese patients. Three patients with ATL were suffering from strongyloidiasis. In one patient it was suggested that ATL developed after horizontal transmission from his wife. In addition, there was a familial case of ATL and HAM/TSP. It seems that in some areas of Colombia, not only HTLV-I infection and HAM/TSP but also ATL are highly endemic.