Abstract We calculate X-ray properties of present-day galaxy clusters from hydrodynamical cosmological simulations of the ΛCDM cosmology and compare these with recent X-ray observations. Results from three simulations are presented, each of which uses the same initial conditions: Non-radiative, a standard adiabatic, non-radiative model; Radiative, a radiative model that includes radiative cooling of the gas; and Preheating, a preheating model that also includes cooling but in addition impulsively heats the gas prior to cluster formation. At the end of the simulations, the global cooled baryon fractions in the latter two runs are 15 and 0.4 per cent, respectively, which bracket the recent result from the K-band luminosity function. We construct cluster catalogues that consist of over 500 clusters and are complete in mass down to 1.18 × 1013, h −1 M⊙. While clusters in the Non-radiative simulation behave in accord with the self-similar picture, those of the other two simulations reproduce key aspects of the observed X-ray properties: namely, the core entropy, temperature-mass and luminosity-temperature relations are all in good agreement with recent observations. This agreement stems primarily from an increase in entropy with respect to the Non-radiative clusters. Although the physics affecting the intracluster medium is very different in the latter two models, the resulting cluster entropy profiles are very similar.