The mandibular dental arches of 45 young adults (mean age = 24.3 years) were analyzed by measuring study casts taken immediately before and twelve weeks after the removal of impacted third molar teeth. Arch length and width were measured at predetermined points on the study casts. The results showed significant buccal movement of mandibular second molar teeth following surgery (p0.01). Bilateral mesioangular impactions were associated with larger increases in mandibular arch width following surgery than unilateral mesioangular impactions or other classes of impaction (p0.05). The greatest amount of tooth movement was observed in cases where mandibular second molars had a pre-operative lingual inclination. It is hypothesised that mesioangularly impacted mandibular third molars can produce sufficient force to displace the adjacent second molar teeth in a lingual direction and when the adjacent third molar is removed this movement is reversed by functional forces.