Previous studies have indicated that the visibility of periapical lesions on a radiograph is determined by erosion of either the cortex or the junction area between cortex and spongiosa. In the present study, artificial lesions, varying in diameter from 1.6 to 4.5 mm, were produced with a round dental bur at 32 sites in 27 blocks from four dried human mandibles embedded in paraffin and radiographed under standardized conditions. The radiographs were evaluated by three observers with differing degrees of radiological experience, and the results expressed as Mean Visibility Index (MVI) and Mean Difference Index (MDI). Six parameters were measured at each site, thickness of the bone, of the cortex and of the spongiosa, density of the spongiosa, cortex/spongiosa ratio, and erosion of the cortex, and their correlation with the two indices determined by stepwise and multiple regression. It was found that MVI was significantly associated not only with the erosion of the cortex but also the density of the spongiosa and the diameter of the lesion. MDI was found to be associated mainly with cortex erosion.