Changes in the periodontal alveolar bone are often evaluated by comparing a series of radiographs taken over time. This investigation used a technique that allowed the image registration to be geometrically standardized each time a radiograph was taken. Radiographs of 24 patients from an ongoing double-blind, clinical study using metronidazole were obtained: (1) before any treatment, (2) at the completion of scaling and root planing and surgery (when performed) and (3) during the maintenance phase. One hundred six (106) paired comparisons were analyzed by subtraction radiography using a computerized system. Of these, 95 (89%) exhibited a minimal degree of geometric distortion and could be successfully substracted. Most areas (67%) showed no change in bone structure following periodontal treatment. Bone gain was noted in 12% of the sites examined, while bone loss was seen in 21% of the sites. This bone loss was statistically associated with sites that had received some form of surgical treatment.