Lateral sinus radiograph is helpful in determining the distances between certain anatomic landmarks in preparation for endonasal endoscopic sinus surgery. However, some misleading factors limit the usefulness of this radiograph. In this study these distances were measured in 49 patients and compared with measurements obtained from another imaging modality, magnetic resonance imaging, which provides more accurate measurements. Results show a discrepancy in the measurements of distances between some of these landmarks that cannot be explained by the factor of magnification alone on the sinus radiographs. Statistical analysis of the results for women revealed a significant discrepancy between the distances from the nasal spine to the midfovea ethmoidalis, the sphenoethmoid junction, and the posterior sphenoid wall as measured on MR images and those as measured on lateral sinus radiographs (P<.05). For men the same was true for the distances from the nasal spine to the midfovea ethmoidalis and the posterior sphenoid wall (P<.05). It is concluded that there should be reservations in the surgical application of data provided by the lateral sinus radiograph before endoscopic sinus surgery.