The phytochemical investigation of the aerial parts of Centaurea deflexa led to the identification of 21 compounds, among which three phenolic acids, one sterol, ten flavonoids, one phenylpropanoid derivative, two lignans and four sesquiterpene lactones. One of the latter compounds was a new, rare active principle (1) having an uncommon 15-nor-guaianolide skeleton. The biological investigation was carried out through a bio-guided assay fractionation of C. deflexa extracts and highlighted an anti-proliferative activity of two sesquiterpene lactones, aguerin B and the newly identified 15-nor-guaianolide (1) against human pancreatic and colonic cancer cells. Of the two compounds, only aguerin B showed to induce apoptotic cell death, confirming the role as pro-apoptotic moiety of the α-methylene-γ-lactone ring present in aguerin B but not in 1. © 2011 Elsevier Masson SAS. All rights reserved.