Mesiodistal crown diameters were measured from twenty pairs of orthodontic pretreatment orthodontic study casts with unilateral peg-shaped maxillary lateral incisors (group I). The control reference sample consisted of twenty pairs of orthodontic study casts with accepted occlusion (group II) matched with the peg-shaped sample according to age and sex. The age range was 14-26 years, both sex were included 11 female and 9 male. The statistical analysis showed an obvious reduction in mesiodistal size of intermaxillary tooth measurements in both sexes in group I. In comparing between the intermaxillary tooth size measurements in females showing peg shape maxillary lateral incisor and control female group, a highly significant reduction in lateral incisors and significant reduction in central incisors, canine and 1st premolar in the upper arch was recorded. Also, a significant reduction in canine and 2nd premolar was observed in the lower arch. However, in comparing between the intermaxillary tooth size measurements in males showing peg shape maxillary lateral incisor and control male group, in the upper arch, a highly significant reduction in lateral incisors and first and second premolars and significant reduction in central incisors, canine was observed. Furthermore, a highly significant reduction was recorded in central and lateral incisors and also first and second premolars, while the canine reported a significant reduction in the lower arch. Significant reduction was found in most of the measurements in female than male in gender comparisons for group 1 subgroups, especially upper premolars The importance of tooth size reduction in orthodontic diagnosis was determined because the relationship between the upper and lower anterior and posterior dentitions is related to orthodontic finishing excellence. The results of the current investigation are of great value in confirming the presence of intermaxillary mesiodistal tooth size reduction in patients showing unilateral pegshaped maxillary lateral incisors. Also this will allow orthodontists to gain insight into the functional and esthetic outcome of a given case.