The acidified gelatin gel technique was used to produce in human teeth in vitro caries-like lesions in relation to unfilled and filled cavities. Results support the suggestion that wall lesion formation in the filled cavity occurs as a result of microleakage of ions along the cavity wall-restoration interface. Differences in appearance of wall lesions in filled and unfilled cavities may be attributed to the presence of the restoration which limits access of ions to the cavity wall.