This study analyzed the length of the root trunk (RT) as well as the width and depth of the prefurcation area (PFA) of human mandibular first molars. Ninety-six human mandibular first molars that were extracted after being observed to be in poor condition both clinically and radiographically were used in this study. A contour-measuring instrument was used, and profiles of the buccal and lingual root surfaces of these teeth were traced on a millimeter-scaled paper. The profiles were recorded from the cementoenamel junction, millimeter by millimeter, up to the entrance of the furcation of each tooth. The width and depth of the PFA as well as the length of the RT were studied. After statistical analysis it was concluded that the mean widths of the PFA on the buccal and lingual surfaces were 3.68 ± 0.66 and 3.48 ± 0.51 mm, respectively; mean depths of the PFA on the buccal and lingual surfaces were 0.89 ± 0.42 and 0.82 ± 0.28 mm, respectively; and mean lengths of the RT on the buccal and lingual surfaces were 3.07 ± 0.82 and 3.54 ± 0.73 mm. Of the samples, the lengths of the RT and the PFA were coincident in 39.58% on the buccal surface and in 46.87% on the lingual surface. There is a concavity (PFA) in the RT region of all mandibular first molars, showing larger width and depth on the buccal surface. The RT was longer on the lingual surface in comparison with the buccal surface.