The maximal interincisal distance added to the vertical overlap is generally used as a measure for temporomandibular joint mobility. However, the length of the mandible also has an influence on this measure. The angle of mouth opening as a measure of temporomandibular joint mobility is independent of the length of the mandible. The aim of the study was to develop a mandibular goniometer to measure the angle of mouth opening and to test its reliability. Ten subjects, two males and eight females (mean age 28.1 years) participated in the study. The measurements were performed during four independent sessions of three measurements each. The results were analysed with ANOVA. Intra-observer variability was found to be 2.0 degrees when the result of all the measurements were analysed, and were found to be 1.2 degrees when the mean value of the sessions were used for analysis. Based on the results of this study it can be concluded that the angle of mouth opening can be measured reliably.