Ultrastructural changes in surface characteristics of enamel white-spot lesions were compared with changes in the adjacent clinically cound enamel after they were etched with 30% phosphoric acid. Ten human permanent first molars exhibiting natural white-spot lesions were used as study specimens. The lesion surfaces and their adjacent sound enamel were etched with 30% phosphoric acid for 60 seconds. Specimens were then evaluated by polarized light microscopy, scanning electron microscopy, and energy-dispersive spectroscopy. The acid etching produced a porous surface on both the white-spot lesion and the surrounding sound enamel. However, the lesion surface appeared to be more resistant to acid and dissolved less than adjacent enamel. This difference in acid solubility produced a steplike appearance between a white-spot lesion and its adjacent enamel surface. Energy-dispersive spectroscopy demonstrated no difference in relative calcium-phosphorus rations among the acid-etched white-spot lesion, acid-etched sound enamel, and unetched sound enamel.