The aim of the present study was to examine the accuracy of linear measurements on radiographs of interproximal bone loss in intrabony defects utilizing the gold standard of surgical 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 was calculated for each radiograph. At the time of surgery, for 34 interproximal intrabony defects, the distances from the cemento-enamel junction (CEJ) to the bottom of the bony defect (BD) and alveolar crest (AC) to BD, and the height of the one-, two- and three-walled section of the intrabony defects were measured. In all radiographs, the linear distance CEJ to BD was assessed. Radiographic and surgical assessments were compared. A stepwise multiple linear regression analysis was used to evaluate factors (angulation difference, patient, surgical measurements) that influenced the discrepancy between radiographic and surgical measurements. The radiographic assessments underestimated bone loss as compared to surgical measurements (1.41+/-2.58 mm).