A twin investigation was carried out to elucidate the influence of heredity in the development of bruxism. The material consisted of 117 pairs of twins of four age classes, average age 12.1 years, in the county of Vasterbotten. Zygosity was determined using blood group serological methods. Bruxism was diagnosed by recording bruxo facets. Occlusal interferences and palpation tenderness in certain masticatory muscles were also recorded.The results show that monozygotic twins have a statistically significant higher frequency of the same facet pattern than do dizygotic twins. It was not established that heredity influences the occurrence of occlusal interferences or muscle tenderness. The study supports the hypothesis that hereditary factors are important to the genesis and pattern of bruxism.