The purpose of this study is to determine what influence the lesion and the premature loss of mandibular 1st deciduous molar have on the eruption of its successor, and the author devided 580 cases of orthopantomograms from age 3 to 9 years old children into 3 groups: Control Group: both side normal Study I Group: one normal and the other lesion. Study II Group: one normal and the other premature loss before 8 years old. and observed the amount of differences and the relative position in eruption between study-tooth successor and normal-tooth successor, The following results were obtained: 1. The differences in eruption between right and left mandibular 1st primary molars of Study Group were greater than those of Control Group. 2. The successors of lesioned-teeth showed more accelerated eruption than the antimeres in 40.9% and more delayed, in 22.7%. 3. The successors of premature lost-teeth showed more accelerated eruption than the antimeres in 61.7% and more delayed, in 4.3%.