Sideroxol (1), a kaurane diterpene which has the ent-7 alpha,18-dihydroxy-15 beta,16 beta-epoxykaurane structure (MW = 320.47, C20H32O3) was obtained from the acetone extract of Sideritis leptoclada plant as well as from some other Sideritis species. It crystallizes in the orthorhombic space group P2(1), 2(1), 2(1) with a = 10.967(3), b = 24.555(5), c = 6.372(4) angstrom, Dc = 1.240 g cm(-3), Z = 4, and refines to R = 0.065 for 721 independent reflections. The skeleton consists of three fused six-membered rings and a five-membered ring with fused epoxide. The six membered rings exhibited slightly distorted chair conformation. In addition to sideroxol, two kaurane and five kaurene diterpenes were isolated from the hexane and acetone extracts of the studied plant.