This study presents a new classification of molar furcation involvement based on the root trunk and horizontal and vertical attachments, and relates them to guidelines in diagnosis, prognosis, and treatment of molar furcation involvements. The types of root trunk were classified on the basis of the ratio of vertical dimension of root trunk to root length as types A, B, and C. Based on th e results of this study, the authors conclude that consideration of the root trunk type as well as the horizontal and vertical attachment loss in the classification of molar furcation involvements may facilitate prognosis, diagnosis, and treatment planning.