Objectives: To provide a description of mesiodistal crown diameters (MD) and tooth-size discrepancy (TSD) of the permanent dentition in patients with thalassemia major (TM) and to compare the results with those of unaffected control group. Study design: The sample consisted of 46 thalassemic patients, 25 males and 21 females aged 7.3 to 23.7 years (mean ± SD = 11.2 ± 3.9 years) and control group matched by age and sex. Dental casts of the participants were measured for MD, using a digital sliding caliper. Descriptive statistics were computed for each individual tooth. Student t-test was used for comparison of mean values between males and females as well as between thalassemic and control groups. The differences between sets of data were evaluated by analysis of variance (ANOVA). Results: All means for MD of thalassemic males and females were smaller than their controls, with 20 of the 24 comparisons being statistically significant (ranged from P<0.05 to P<0.001). In both thalassemic and control groups, males exhibited significantly larger MD than females in most instances. Canines displayed the most sexual dimorphic teeth in the dentition. Lateral incisors showed the greatest variability indicated by the coefficient of variation, while the first molars were the least variable teeth (7.6% vs. 5.3%). There were no statistically significant differences in the anterior and overall tooth-size discrepancy ratios between sexes or between thalassemic and control groups. The mean anterior ratio (79.5%) and overall ratio (92.4%) of the control group were significantly larger than of Bolton ratios; P<0.001 and P<0.05, respectively. Conclusions: The present study demonstrated that thalassemic males and females exhibited significantly smaller MD than the control group. The TSD ratios in both thalassemic and control groups were significantly larger than those of Bolton sample. These findings should be taken into account when planning orthodontic treatment for thalassemic patients.