The aetiology of mouth ulcers is uncertain, and prior research has indicated both environmental and genetic factors. In this study, information on mouth ulcer incidence was collected for 290 twin pairs--127 monozygous (MZ) and 163 dizygous (DZ)--and their parents, a total of 1,160 people. Self-reported data on mouth ulcer incidence were available for the twins, and in each family the mother also reported on the mouth ulcer incidence in the twins, the twins' father, and herself. A structural equation model--combining a measurement model, a rater bias model, and a model including genetic and environmental influences--was used to explain variation in mouth ulcers. The fitted model explained the variation in a latent phenotype of mouth ulcer incidence for the twins in terms of an additive genetic factor (64%), a common environment factor (26%), and a specific environment factor (10%). The mothers' ratings showed a significant positive bias.