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The melampolide-class sesquiterpene lactone 11βH,13-dihydro-14-hydroxy-(1,10)-cis-parthenolide, C15H22O4, 1, crystallizes in orthorhombic space group P212121 with a = 7.435(2), b = 12.678(2), c = 14.991(2) Å, V = 1413.1(8) Å3, and Z = 4. The aldehyde analog, C15H20O4·H2O, 2, crystallizes as the monohydrate in orthorhombic space group P212121 with a = 9.061(2), b = 9.295(2), c = 17.761(4) Å, V = 1495.8(9) Å3, and Z = 4. The 10-membered rings of the two molecules adopt approximate chair-boat conformations. In both compounds, the double bond at C1=C10 position of the cyclodecene ring is Z, while the C4=C5 is epoxidized with an E-configuration. The C15 methyl is β and H5 is α. The lactone ring is trans fused at the C6 and C7 positions to the 10-membered ring. The C13 methyl group has an α orientation in both molecules.