Mandibular range of motion (ROM), vertical and horizontal, is often measured as a part of a diagnostic assessment of temporomandibular joint disorders. In the literature, a fixed ratio between the vertical and the horizontal ROM has been suggested, i.e. 4:1. The ratio is frequently used to predict the vertical ROM on the basis of the horizontal ROM and vice versa. However, no scientific data are available to substantiate the ratio suggested. The aim of this study was to determine whether the fixed ratio of 4:1 exists and, if so, whether this ratio has a predictive value. Vertical and horizontal mandibular ROM was measured in 91 healthy subjects, 59 women, and 32 men (mean age 27.2 years, s.d. 7.4 years) using vernier callipers. We found a mean ratio between vertical and horizontal ROM ranging from 6.0:1-6.6:1. On an individual basis the ratio ranged from 3.6-15.5. Correlations between vertical and horizontal ROM were weak. On the basis of the results on this study it is concluded that the ratio between vertical and horizontal ROM is approximately 6:1 rather than 4:1, and that the ratio has poor predictive value.