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The aim of the present study was to compare linear measurements of interproximal bone loss on digitized radiographic images after application of different filters to the gold standard of intrasurgical measurements. Immediately before surgery, 50 radiographs of 50 periodontally-diseased teeth exhibiting interproximal bone loss, were obtained by a standardized technique in 33 patients. Intrasurgically, the distances from the cementoenamel junction (CEJ) to the alveolar crest (AC) and from the CEJ to the deepest extension of the bony defect (BD) were assessed. All radiographs were digitized with a flatbed scanner (resolution: 600x1200 dpi). Using the FRIACOM-soft-ware, the linear distances CEJ to AC and CEJ to BD were measured at 50 interproximal lesions from the digitized but unchanged radiographic images and also after use of 7 different basic image-processing modes (enhancement of contrast [dynamics], inversion, high-pass, enhancement of gray-level differences, mean value, histogram correction, spreading of grey values) with 11-fold magnification. Neither the measurement of the distance CEJ to AC on the unchanged images nor assessments with any of the filters revealed significant differences from the gold standard. Multivariate analysis of variance showed the respective filter (p=0.009), intrasurgical and radiographic assessments (p