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This study analyzed the root trunk (RT) and the pre-furcation area (PFA) of mandibular first molars. Thirty lower first mandibular molars extracted due to advanced periodontal disease were evaluated in a high-energy spiral computerized micro-tomography. Two gutta-percha markings on the cementoenamel junction (CEJ) and at the furcation entrance (FE) at buccal and lingual surfaces served as reference points for measurements of RT length, and PFA width and depth, at the levels of CEJ, 1 mm apical to CEJ, 2 mm apical to CEJ, and at the FE. The mean RT length was 2.49 mm at buccal and 3.18 mm at lingual sides. The mean widths of the PFA at CEJ, at 1 and 2 mm apical to CEJ, and at FE were 2.9, 3.4, 3.9 and 4.3 mm, respectively, while the mean depths were 0.19, 0.32, 0.57 and 1.1 mm, respectively. The PFA coincided with CEJ in 10 buccal and 10 lingual surfaces, representing 33.33% of the sample. There was a negative correlation between RT length and PFA dimensions. This study concludes that the RT length was smaller than previous studies. From the CEJ up to the furcation entrance, the PFA showed a progressive increase in width and depth. The coincidence of the PFA area beginning at the CEJ in 1/3, and the negative correlation between RT length and PFA dimensions may represent greater risk factor for the early development of furcation lesions. [ABSTRACT FROM AUTHOR]