The purpose of this study was to determine the effect of initial periodontal therapy on crestal bone remineralization with the use of a digital subtraction method for quantitative analysis. Fifteen crestal sites randomly selected from five patients were examined at the initial visit, after completion of oral hygiene instruction, and 1, 3, and 6 months after scaling and root planing with standardized radiographs. The radiographs were digitized by means of a drum-scanning microdensitometer with a sampling aperture of 50 microns producing 256 gray levels and corrected for difference of contrast between image pairs. Results were as follows: (1) statistically significant changes were observed even 1 month after the therapy and (2) the extent of changes was quantitatively expressed and monotonically increased with time. These results indicate that subtraction radiography may be a reproducible and quantitative method for the evaluation of periodontal therapy.