A 60-year-old woman from the town of Mashhad in northeastern Iran developed cardiac failure due to aortic and mitral regurgitations which needed cardiac valve replacement. Histopathological study of the valves revealed a T-cell non-Hodgkin's lymphoma. Blood examination showed leukemic features with 32% of abnormal white blood cells. Human T-cell leukemia/lymphoma virus type I (HTLV-I) antibodies were present in the serum and the specific env HTLV-I sequences were detected in the DNA extracted from the valves and peripheral blood mononuclear cells (PBMC) using polymerase chain reaction technique. Clonal integration of two HTLV-I copies was found in both the valves and PBMC DNA, thus the diagnosis of adult T-cell leukemia/lymphoma (ATL) was established. In contrast to the acute life-threatening cardiac localization, our case met the diagnostic criteria of chronic ATL, this was confirmed by favorable evolution without chemotherapy during the 24 months after diagnosis. According to our knowledge, this is the first report of an isolated lymphomatous cardiac valve involvement, without other cardiac abnormalities. It seems important to underline that the patient originated from Iran where endemicity of HTLV-I has only recently been discovered.