Phytochemical investigation of the dried biomass of Asclepias syriaca afforded five new compounds (1-5), along with 19 known structures. Overall, the secondary metabolites isolated and identified from this plant showed a wide structural diversity including pentacyclic triterpenes, cardiac glycosides, flavonoid glycosides, lignans, a phenylethanoid, and a glycosylated megastigmane. In addition, the isolates were tested against the cancer breast cell line Hs578T, and those showing IC(50) values lower than 50 μM (1 and 6-9) were further investigated in three additional breast cancer cell lines (MCF-7, T47D, and Sk-Br-3) and the normal breast cell line Hs578Bst.