Objective: To study the reliability of the Tanaka-Johnston mixed-dentition analysis in Turkish individuals and develop a new population-adjusted regression equation for predicting the size of unerupted canines and premolars. The new regression equations were tested to establish their validity in Turkish children. Methods: The mesiodistal dimensions of teeth on study casts of 12- to 14-year-old Turkish children were measured. The actual tooth measurements were then compared with predicted values using the Tanaka-Johnston equation. Subsequently, the new regression equations were tested in a group of 50 individuals to determine their validity. Results: The Tanaka-Johnston regression equations overestimated the actual mesiodistal widths of permanent canines and premolars. With the new regression equations, no statistically significant differences were observed between actual and predicted widths. Conclusion: The Tanaka-Johnston prediction method is not accurate when applied to a Turkish population. The new equations are reliable for predicting the width of unerupted canines and premolars in Turkish individuals. World J Orthod 2009;10:135­–140.