Pocket depth, attachment level and bone level assessments were carried out using flexible splints to produce readily identifiable reference points and to standardize the probing spot and the direction of probe insertion. The pocket depth and attachment level measurements were carried out twice at intervals of 3 weeks, both before and 3 months after periodontal treatment. The level of alveolar bone was measured by transgingival probing ("sounding") and again following elevation of a mucoperiosteal flap. The measurements were made with a periodontal probe to the nearest higher millimeter. Complete agreement was found between the first and second measurements of pocket depths and attachment level for approximately 60% of the examined surfaces, both before and after periodontal treatment. A deviation of 1 mm or less was found for approximately 95% of the surfaces, and the difference between the first and second measurement never exceeded 3 mm. When transgingival probing measurements were compared to the measurements of the periodontal bone level assessed after elevation of a mucoperiosteal flap, complete agreement was found for 60% of the surfaces, and a deviation of 1 mm or less was found for 90% of the surfaces. No discrepancy exceeding 3 mm was observed. The results of this study indicate that readily identifiable reference points can be produced by flexible splints in assessments of pocket depth, attachment level and bone level alterations in studies on the effect of periodontal treatment.