The aim of this investigation was to determine the relationship between loss of radiographic crestal bone height and probing attachment loss. As part of this aim, we are introducing a new method for determination of a threshold for significant probing attachment loss which takes into account the error contributed by alteration in tissue tone. 57 adults with established periodontitis were selected. Radiographs were taken using the Rinn alignment system. Crestal change was determined from bone height measurements on digitized images of pairs of radiographs using the "side-by-side" technique of analysis developed recently by us. Probing attachment loss was measured using the Florida electronic probe system. Radiographic and probing measurements were made at baseline and after 1 year. No treatment was given during this period. A direct and significant relationship was observed between radiographic bone loss and probing attachment loss on a site basis (p = 0.0001, r2 = 0.018) and between subject means (p = 0.0014, r2 = 0.16). Radiographic and probing attachment change at all categories of sites, dichotomously classified as to not changing or loosing indicated 13% of sites were loosing by measurement of radiographic change and 9.6% were loosing by measurement of attachment change. Concordance in radiographic and attachment level change was found in 82% of sites examined. The relative diagnostic import of change in probing attachment or change in radiographic bone height requires treatment outcome studies based on use of diagnostic information of the 2 measuring techniques used singly and in combination.