Background The prevalence of malocclusion in modern population is higher than that in the excavated samples from the ancient times.Presently,the prevalence of juvenile malocclusion in the early stage of permanent teeth is as high as 72.92% in China.This study aimed to observe and evaluate the prevalence and severity of malocclusions in a sample of Xia Dynasty in China,and to compare these findings with the modern Chinese population.Methods The material consisted of 38 male and 18 female protohistoric skulls of Xia Dynasty 4000 years ago.Of 86 dental arches,29 cases had the jaw relationships.Tooth crowding,diastema,individual tooth malposition and malocclusion were studied.Results Of the samples,23.3% showed tooth alignment problems including crowding （8.1%）,diastema （9.3%）,and individual tooth malposition （5.8%）.The prevalence of malocclusion was 27.6%,mainly presented as Angle Class Ⅰ.Conclusions It is indicated that over thousands of years from Neolithic Age （6000-7000 years ago） to Xia Dynasty （4000 years ago）,the prevalence of malocclusion did not change significantly.The prevalence of malocclusion of Xia Dynasty samples was much lower than that of modern population.
