Background: The space available for an unerupted mandibular third may depend on the choice of premolar extracted. 

Aims: To investigate the effects of orthodontic treatment and premolar extractions on the inclinations of the mandibular third molars and the space available for their eruption, and to compare these changes with a nonextraction group. 

Methods: The pre- and post-treatment panoramic radiographs of 54 subjects (20 males, 34 females) were used. Eighteen of these subjects had the four first premolars extracted, 16 subjects had four second premolars extracted and 20 subjects were treated nonextraction. Changes in the inclinations and spaces available for the unerupted third molars were compared. 

Results: In the nonextraction group the third molars uprighted approximately 1 degree and in the second premolar extraction group the third molars uprighted 10 degrees. The spaces available for the third molars increased significantly in the first and second premolar extraction groups as compared with the space available in the nonextraction group. 

Conclusions: Orthodontic treatment and extraction of the second premolars improved the inclinations of unerupted third molars and the space available for their eruption into the arch. The changes in inclination and eruption space were less marked following first premolar extractions. (Aust Orthod 2010; 26: 160-164)