Limited mandibular movement is one of the cardinal signs of temporomandibular disorders, and is usually assessed by measurement of mouth opening. Linear measurement of mouth opening does not discriminate between diagnostic groups and has been shown to be influenced by gender, age and ramus length. A new temporomandibular opening index, however, can differentiate between diagnostic groups and is not influenced by the above variables. When patients with a myogenous temporomandibular disorder were examined, the temporomandibular opening index distinguished two subgroups, the first with values about the lower quartile and the second with values about the upper quartile. A similar identification of two subgroups was seen in electromyographic studies of myogenous temporomandibular disorder patients (Visser, A., Kroon, G.W., Naeije, M. & Hansson, T.L. (1995) Journal of Oral Rehabilitation, 22, 429).