Observations are reported on the effect of continuous CO2 laser irradiation on the structure and microhardness of AlSI 1045 steel. In the case of isolated beam passes, martensite formed in the melt zone and in former pearlite regions of the austenitization zone exhibits very high Vickers hardness values (HV 750 and 900, respectively). However, in the case of contiguous or partly overlapping passes a zone of tempered martensite with hardness down to HV 400 forms behind each pass, thus resulting in a seesaw hardness distribution across the processed surface. It has been found that a drastic decrease in the size of the laser-affected region occurs when the beam power density exceeds a certain threshold level.