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Precise reproduction of probe placement, probing force and probing angulation may influence the reliability of replicate gingival attachment level measurements (GAL). Reproducibility of GAL measurements was determined with the Toronto automated periodontal probe (TAPP) in which a novel electronic guidance system was incorporated to improve the precision of probing angulation. Bench testing of forces produced at the probe tip was performed with an electronic balance. Reproducibility and precision of force generation were tested in a variety of instrument configurations using laboratory models. The data indicated that highly precise and reproducible probing forces can be generated with the TAPP over a large (10 to 90 g) probing range. Instrument precision was +/- 0.27 mm when probing forces of 40-60 g were used. The guidance system provided accurate and reproducible measurements of the probe handle in both roll and pitch axes when compared with precisely machined wedges (R = 0.99, roll; R = 0.98, pitch). Deviation of probing angulation greater than 5 degrees on replicate measurements of GAL was associated with significant alterations in the location of circumferential probe placement and in the size of the difference between GAL measurements (delta GAL). Clinical assessment of probing angulation in 6 patients demonstrated that delta GAL was 0.40 +/- 0.05 mm when probing angulation deviation was less than 5 degrees but increased to 0.96 +/- 0.11 mm when deviations exceeded 5 degrees. Taken together, these data indicate that the regulation of probing angulation in clinical measurement of GAL with the TAPP is an important determinant of the reproducibility of periodontal probing.