The determination of the tooth size-arch length discrepancy in the mixed dentition requires an accurate prediction of the mesiodistal widths of the unerupted permanent teeth. Three approaches have been used to estimate the mesiodistal crown widths of unerupted canines and premolars: (1) measurements from erupted teeth, (2) measurements from radiographs, and (3) a combination of measurements from erupted teeth and from radiographs of unerupted teeth. The purpose of this study was to compare two mixed dentition prediction methods that do not require the use of periapical radiographs of the unerupted permanent teeth. The two methods compared were the Tanaka/Johnston and the Boston University Prediction approaches. Records on 33 male and 22 female subjects who were participants in the Iowa Longitudinal Growth Study were used in the present study. The findings indicated that on the average the Tanaka/Johnston approach slightly overestimated the tooth size of the unerupted teeth (mean = 1.1 +/- 0.9 mm). On the other hand, the Boston University approach slightly underestimated the tooth size of the unerupted teeth (mean = -0.1 +/- 1.2 mm). The findings further indicated that there were statistically significant correlations between the predicted and actual tooth size. In general the r values were higher for the Tanaka/Johnston method than with the Boston University approach. The error involved in the use of the predicted equations was expressed as the standard error of the estimate. The present findings indicated that the standard error of the estimate for Tanaka/Johnston prediction equations ranged between 0.67 and 0.92 mm and the corresponding values for the Boston University equations ranged between 0.92 and 1.02 mm. Depending on the stage of dental development, i.e., which deciduous and permanent teeth are present, the Tanaka/Johnston approach can be used when the four mandibular incisors have completely erupted, whereas the Boston University approach can be used when all the deciduous canines and first molars are still present.