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 The present work is devoted to the investigation of middle-cycle fatigue(» 105 cycles). Smooth specimens made of 35CD4 quenched and tempered steel wereloaded in fully reversed plane bending. Temperature fields were recorded with aninfrared camera. Our experimental results show that the local heating of metal underfatigue loading is a sensitive and accurate enough manifestation of damage initiation.The appearance of mesoscopic defect structures leads to the time correlated thermalbehavior of adjacent points in the recorded temperature field. It is shown that the timeevolution of the spatial standard deviation of the temperature can be used to investigatethe defect collective properties, the damage localization and to monitor both theinitiation and the current location of fatigue crack tip.