The isolation and structure characterization of a dienone from the roots of Echinacea pallida, namely (8Z,11Z)-pentadeca-8,11-dien-2-one, are described here. To assess the configuration of this secondary metabolite, the stereoselective total synthesis of the two isomeric forms, 8Z,11Z)- and (8Z,11E)-pentadeca-8,11-dien-2-one, was undertaken and the structure elucidation of the naturalcompound was unambiguously carried out. The cytotoxic activity of both isomers was also evaluated on a human T cell leukaemia cancer line (Jurkat cells). The results indicated that these compounds exert a dose-dependent cytotoxicity with a medium-level potency on the tested cell line.