Adult T-cell leukemia/lymphoma (ATLL) is an aggressive mature T-cell neoplasm caused by infection with the Human T-cell Lymphotropic Virus Type 1 (HTLV-1). Cardiac involvement in patients with ATLL is infrequent, and when it happens it is usually seen in aggressive ATLL subtypes. However, ATLL presenting as isolated cardiac valve involvement is extremely rare. To date, only three histologically proven cases of ATLL with isolated cardiac valve involvement have been reported. Herein, we describe a 61-year-old Peruvian man who presented heart failure symptoms secondary to progressive cardiac valve infiltration. The patient underwent mitral valve replacement with a mechanical prosthesis. Histopathological evaluation of the resected valve revealed leaflet thickening with a nodular appearance due to fibrous tissue containing atypical T-lymphocytes with Foxp3 expression, infiltrating all layers...