Distal caries of the second molar is common indication for the mandibular third molar surgery and there are no universally acceptable predictive criteria for distal caries of the second molar. To analyze the correlation of the distal caries of the second molar and the eruption state of the mandibular third molar using panoramic radiographs statistically and propose the acceptable guideline for preventive extraction of the mandibular third molar. 786 patients who were extracted the mandibular third molar from 2002 to 2006 at Samsung medical center were examined. The presence and absence of distal caries of mandibular second molar, age, gender, angulation, impaction degree, distance between distal cementoenamel junction of the second molar and mesial cementoenamel junction of the mandibular third molar were assessed. 79.6% of third molars had a mesial angulation of between $40^{circ}$ and $80^{circ}$. The mean age of third molar removal for distal caries of second molar was $33.86{pm}9.81$. The prophylactic removal of a mesio-angular third molar about $40^{circ}$ and $80^{circ}$ could prevent distal cervical caries forming in the mandibular second molar.