A 2.29 GHz very long baseline interferometry all-sky survey of ultra-compact radio sources due to Preston et al. has formed the basis of a number of cosmological investigations, which examine the relationship between angular size and redshift. Here I use a sample of 468 such sources with 0.5 < z ≤ 3.787 to investigate the isotropy of the Universe. The sample is divided into hemispherical sub-samples, over an all-sky 5° × 5° array, each of which is allowed to determine a value of Ωm, assuming that we are living in a spatially flat homogeneous isotropic Λ cold dark matter model. If we regard the latter as a null hypothesis, then it fails the test – the results show significant anisotropy, the smallest value of Ωm being towards (l, b) = (253.9, 24.1)°, and the largest in the opposite direction. This is close to the cosmic microwave background dipole axis, but in the obverse sense. This is interpreted as meaning that the Universe is not spatially homogeneous on the largest scales, and is better represented at late times by a spherically symmetric model with a density enhancement at its centre.