In this investigation, the change in microhardness of enamel has been correlated with the decrease in calcium concentration after slow decalcification and etching. In three different directions, with respect to the orientation of the enamel prisms, microhardness measurements did not indicate a mechanical anisotropy. Microhardness experiments made in longitudinal sections of the tooth show that the surface layer covering the lesion possesses a significantly greater hardness value than within the lesion. The local calcium content and the duration of decalcification are correlated with the microhardness of decalcified enamel. In etched enamel, the microhardness in the acid-penetrated area and the calcium content, were at a lower level compared with untreated enamel. In the lesion, hardness was almost negligible; the hardness in the etched area was reduced to approximately 50% of its original value.