The aim of the present study was to compare computer-assisted linear measurements of interproximal intrabony defects on radiographs using two different methods with the gold standard of intrasurgical measurements. In 22 patients with untreated advanced periodontal disease, 33 standardized radiographs were taken presurgically. The horizontal and vertical angulation difference of the central beam from the orthoradial projection and radiographic magnification were calculated for each radiograph. At the time of surgery, for 34 interproximal intrabony defects, the distance from the cemento-enamel junction (CEJ) to the bottom of the bony defect (BD) was measured. The distance CEJ to BD was assessed on the radiographs by two examiners that were blinded for the clinical parameters each using a different computer-assisted system (TSK: method 1; EH: method 2). Both radiographic evaluation methods underestimated interproximal bone loss statistically significantly [method 1: 0.70 ± 2.31 mm (p=0.006); method 2: 1.41 ± 2.58 mm (p=0.003)]. The difference in underestimation of bone loss between method 1 and 2 (0.71 ± 2.31 mm) failed to reveal statistically significance (p>0.05). A multivariate analysis confirmed this observation and failed to show any influence of projection geometry on underestimation of bone loss. Both computer-assisted methods used to measure interproximal bone loss on radiographs in this study underestimated the amount of bone loss statistically significantly. Schlagwörter: computer-assisted radiograhic analysis, intrabony defects, periodontal diagnosis, linear measurements