This study aims to quantify the amount of sexual dimorphism in primary and permanent tooth crown size in a sample of Australian twins and to explore the role of hormonal factors in human dental development. We hypothesise that the magnitude and patterning of sexual dimorphism within and between the primary and permanent dentitions of the same individuals will reflect associations between the timing of initial stages in the process of odontogenesis and the timing of hormonal surges during pre-natal and peri-natal development. Serial dental models of the primary, mixed and permanent dentitions of 88 males and 91 females from monozygotic and dizygotic same-sex twin pairs were used. Mesiodistal crown diameters (MD), buccolingual crown diameters (BL), crown heights (CH), and intercuspal distances (IC) of all primary teeth and the permanent central incisors, lower lateral incisors, canines, second premolars, first and second molars were measured to an accuracy of 0.1mm using a 2D image analysis system. Mean values, standard deviations, coefficients of variation, percentages of sexual dimorphism, and correlation coefficients were calculated for all variables. Overall, males presented larger tooth crown dimensions than females, with the primary dentition displaying less sexual dimorphism compared with the permanent dentition. Intercuspal distances tended to show the least sexual dimorphism whereas crown heights showed the most, reflecting differences in the timing of formation of these dimensions during odontogenesis. These results are consistent with some hormonal influence during tooth development, but further studies of twins, including opposite-sex dizygotic pairs, are needed to clarify the nature of this hormonal effect.