Mandibular movement may be assessed by measuring maximum mouth opening. This is a linear measurement from the maxillary to the mandibular incisai edge and is affected by gender, age and ramus length. It cannot be used to classify patients. The Temporomandibular Opening Index (TOI) appears to be independent of these variables and has been shown to differ for different categories of patients with Temporomandibular Disorders (TMD). This study examined the TOI in a group of 11 patients with closed lock and a control group of II individuals with no signs or symptoms of TMD. The group with closed lock had a significantly different TOI from that of the control group (P < 0.001) according to the Mann-Whitney U-test. This suggests that the TOI may have greater diagnostic value than linear mouth opening.