The aims of this study were to estimate the average maximum mouth opening and range of mouth opening in a representative sample of the Irish adult population, to compare mouth opening in patients with and without signs or symptoms of temporomandibular joint (TMJ) dysfunction syndrome and to investigate the association between mouth opening and stature among populations. As part of the Irish National Survey of Adult Oral Health, maximum mouth opening was measured and questionnaire data collected for 1513 adults, aged 16-99 years. The average maximum mouth opening was 43 mm for males and 41 mm for females. Mouth opening was found to reduce with age, independent of gender. No link was found between reduced mouth opening and TMJ dysfunction or between mouth opening and stature in populations.