The crystal structure of the title compound, C(17)H(28)O(5), reported here is a polymorph of the structure first reported by El-Feraly, Al-Yahya, Orabi, McPhail & McPhail [J. Nat. Prod. (1992). 55, 878-883]. It is a derivative of the anti-malaria compound artemisinin and consists primarily of three substituted ring systems fused together. A cyclo-hexane ring (distorted chair conformation) fused to a tetra-hydro-pyran group (distorted chair) is adjacent to an oxacyclo-heptane unit containing an endo-peroxide bridge, giving the mol-ecule its particular three-dimensional arrangement. The crystal packing is stabilized by inter-molecular C-H⋯O inter-actions between an O atom from the endo-peroxide bridge and H atoms from both the cyclo-hexane and seven-membered oxacyclo-heptane fused rings, as well as between an O atom and H atom from adjacent tetra-hydro-pyran rings. The two polymorphs have the same space group and similar cell parameters for the a and b axes, but significantly different values for the c axis.