Objective: To investigate the reliability of length measurements of the mandible by comparing orthopantomograms (OPTs) with lateral cephalograms. Design: Observational study. Setting: OPTs and lateral cephalograms were taken of 20 human dry skulls. Four orthodontists and four maxillofacial surgeons located landmarks on all radiographs using a computer program for cephalometric measurements. Intraobserver and interobserver variability in locating landmarks was assessed, as well as positioning of the skulls prior to radiography between the x-ray assistants. Magnification differences between the left and right side of the mandible on the OPT were determined for five skulls. Kappa statistics were used to calculate the intraclass correlation coefficient for intraobserver and interobserver differences. An F test was used to assess differences between methods and between type of observer. Results: No significant differences were found in the magnification factor of the left and right side of the mandible. Compared with a lateral cephalogram, the OPT had comparable reliability in measuring mandibular distances condylion-gonion, gonion-menton, and condylion-menton. No significant differences were observed between the x-ray assistants in taking the OPTs and lateral cephalograms or in repositioning the skulls. Significant differences were found between orthodontists and maxillofacial surgeons for landmark measurements. Conclusion: An OPT is as reliable as a lateral cephalogram for linear measurements of the mandible (condylion-gonion, gonion-menton, and condylion-menton).