Introduction: Missing or peg-shaped maxillary lateral incisors can affect the mesiodistal dimensions of the dentition. Methods: The pretreatment casts of patients with bilateral (BMLI, n = 35) or unilateral (UMLI, n = 29) missing maxillary lateral incisors or bilateral peg-shaped maxillary lateral incisors (BPLI, n = 16) were evaluated. The mesiodistal widths of all permanent teeth except the second and third molars were measured, and the sums of total and quadrant tooth widths were calculated. The results were compared with a control group (n = 32) with normal maxillary lateral incisors. A quadrant analysis was performed by comparing the quadrants with lateral incisor agenesis, peg-shaped lateral incisors, and normal lateral incisors with each other. Statistical analysis was performed by 1-way analysis of variance followed by Tukey and Tamhane tests. Results: In the BMLI and BPLI groups, significant reductions in individual tooth widths and total tooth widths were seen. The size discrepancies were the greatest in the maxillary central incisors and decreased gradually from the incisors to the molars. Compared with the control group, the tooth-size discrepancy was more pronounced in the BPLI group followed by the BMLI and UMLI groups. Due to the variability in clinical presentations of the UMLI group, the association with the reduction of the tooth sizes was not significant. But the quadrant analysis showed that in quadrants with a missing or peg-shaped lateral incisor, the teeth were significantly narrower than in quadrants with normal incisors; this indicates the importance of the contralateral side in unilaterally affected patients during analysis. Conclusions: Bilateral missing or peg-shaped maxillary lateral incisors were significantly associated with reduction of individual mesiodistal tooth widths as well as their total and quadrant sums. Patients with a unilaterally missing lateral incisor should be analyzed individually with quadrant analysis, since the variability in the contralateral side would affect the traditional analysis results.