We present a comprehensive study of field-induced anisotropy in ferrofluids using two-dimensional small-angle x-ray scattering. On application of a homogeneous magnetic field, magnetite particles form anisotropic clusters with preferred orientation parallel to the field. The orientation of single nonspherical particles contributes little to structural anisotropy, since their effective axis ratio is only about 1.05. We determine the average radius of particles in clusters to be of the order of the mean particle radius in the ferrofluid, i.e., 4–5 nm. Therefore, particles of all sizes contribute to cluster formation. Since the thermal energy exceeds the magnetic dipole-dipole interaction of small particles, thermally stable clusters cannot form without an additional interaction energy. We show that van der Waals interaction yields a significant contribution that cannot be neglected in polydisperse systems.