The determination of a tooth-size to arch-length discrepancy in the mixed dentition requires an accurate prediction of the mesiodistal widths of the unerupted permanent teeth. This is an essential factor in treatment planning. The aim of this study was to validate Tanaka and Johnston's analysis on 600 Syrian patients aged 14-22 years. Tanaka and Johnston's tables, equations, and approximations were modified in order to improve the accuracy of the prediction. The correlation coefficients found between the size of the permanent mandibular central incisors and maxillary first molars (31, 41, 16, and 26) and the maxillary and mandibular canines and premolars were high (r = .72 and .74, respectively). New, more accurate prediction tables applicable at earlier ages, and new regression equations were constructed. In addition, new easier approximations were developed to allow the prediction of the size of the unerupted maxillary canines and premolars by adding 6 mm to the half-widths of teeth 31, 41, 16, and 26. The analogous prediction of the size of the unerupted mandibular canines and premolars was obtained by adding 5.5 mm to the half-widths of same teeth, 31, 41, 16, and 26.