An experimental study on the grain growth of a medium-carbon V-Ti microalloyed steel with two levels of AIN has been carried out. A system to study grain-size distributions in order to detect the abnormal grain growth has been proposed. Log-normal distributions were verified and then properties of normal distributions were applied to distinguish normal and abnormal grains. The benefits of working with the relative difference (RD) of grain size in order to compare the grain-growth behaviour have been discussed. It was experimentally concluded that abnormal growth appears when RD is larger than 2.5. From the results a map of abnormal grain growth against time and temperature could be plotted. It was concluded that abnormal grain growth is due to the AlN dissolution when the above maps are correlated with the theoretical volume fraction of precipitates. The importance and effect of heating rate have also been shown : high heating rates can produce abnormal growth at higher temperatures than those of the equilibrium dissolution.