Magnetic frustration in three dimensions (3D) manifests itself in the spin-1 2 insulator Li2CuW2O8. Density-functional band-structure calculations reveal a peculiar spin lattice built of triangular planes with frustrated interplane couplings. The saturation field of 29 T contrasts with the susceptibility maximum at 8.5 K and a relatively low Néel temperature TN 3.9 K. Magnetic order below TN is collinear with the propagation vector (0, 1 2 , 0) and an ordered moment of 0.65(4) µB according to neutron diffraction data. This reduced ordered moment together with the low maximum of the magnetic specific heat (C max /R 0.35) pinpoint strong magnetic frustration in 3D. Collinear magnetic order suggests that quantum fluctuations play crucial role in this system, where a noncollinear spiral state would be stabilized classically. Introduction. Magnetic frustration, the competition of exchange couplings between localized spins, has broad implications for ground states, excitation spectra, and low-temperature properties. Prominent manifestations of the frustration include peculiar behaviors of spin ice [1], the formation of quantum spin liquids [2], and non-collinear magnetic structures that give rise to spin chirality and strong magnetoelectric coupling [3]. The properties of frustrated magnets change drastically depending on the dimensionality of the spin lattice. Rigorous mapping between theory and experiment requires that both magnetic models and real materials are relatively simple. In this context, the case of isotropic (Heisenberg) exchange on a three-dimensional (3D) lattice of quantum spin-