We present a stability analysis of the two-dimensional t − t ′ Hubbard model for various values of the next-nearest-neighbor hopping t ′ , and electron concentrations close to the Van Hove filling by means of the flow equation method. For t ′ ≥ −t/3 a d x 2 −y 2 -wave Pomeranchuk instability dominates (apart from antiferromagnetism at small t ′ ). At t ′ < −t/3 the leading instabilities are a g-wave Pomeranchuk instability and p-wave particle-hole instability in the triplet channel at temperatures T < 0.15t, and an s * -magnetic phase for T > 0.15t; upon increasing the electron concentration the triplet analog of the flux phase occurs at low temperatures. Other weaker instabilities are found also.PACS numbers: 71.10. Fd, 71.27.+a, 74.20.Rp, 75.10.Lp In recent years the two-dimensional (2D) Hubbard model has been used [1,2] as the simplest model which maps the electron correlations in the copper-oxide planes of high-temperature superconductors since experimental data suggest that superconductivity in cuprates basically originates from the CuO 2 layers [3]. Although in the high-temperature cuprate superconductors electron-electron interactions are strong some important features of these systems (in particular, antiferromagnetic and d-wave superconducting instabilities) are captured already by the 2D Hubbard model at weak to moderate Coulomb coupling.Apart from the antiferromagnetism and d x 2 −y 2 -wave superconductivity mentioned above (for review see [1, 2, 4] and references therein), a few other instabilities related to symmetry-broken states [5,6,7,8,9,10,11] in the 2D t − t ′ Hubbard model with next-nearest-neighbor hopping t ′ have been reported recently. Specially, much interest of researchers has been attracted by the case when the Fermi surface passes through the saddle points of the single particle dispersion (Van Hove filling). One of the instabilities found in such a case is a d-wave Pomeranchuk instability breaking the tetragonal symmetry of the Fermi surface, i.e. a spontaneous deformation of the Fermi surface reducing its symmetry to orthorhombic. It has been recently observed for small values of t ′ from renormalization group calculations by Halboth and Metzner [5]. They argued that the Pomeranchuk instability occurs more easily if the Fermi surface is close to the saddle points with a sizable t ′ (reducing nesting which leads to antiferromagnetism). However, within their technique it is difficult to compare the strength of the Fermi surface deformation with other instabilities and to conclude which one dominates. The authors of Ref.[10] have investigated the interplay of d-density wave [12,13] and Fermi surface deformation tendencies with those towards d-wave pairing and antiferromagnetism by means of a similar temperature-flow renormalization group approach. They have found that the d-wave Pomeranchuk instability never dominates in the 2D t − t ′ Hubbard model (even under the conditions mentioned above). On the other hand, Vollhardt et al. [14] showed that the t ′ -hopping term destroys the an...