Background: Volchansky and Cleaton-Jones (IADR Brisbane, 2006) reported an increase in furcation lesion (FL) prevalence from 30 years onwards with the rate on the buccal (B) significantly greater than on the lingual (L) sides of the mandibular first molar (M1). Objectives: To measure root trunk size in relation to furcation lesions (FL) of M1 from 50+ years. Methods: 46 mandibles in the Raymond Dart Collection of the School of Anatomical Sciences at the University of the Witwatersrand fulfilling the following inclusion criteria were the study sample: aged 50y + (mean 56.3y sd 5.1y), from Black Africans, with no carious or restored teeth, with PM2 and M2 present. Standardised photographs (P) of occlusal, right and left buccal and lingual views, were made with a ball bearing placed on M2 to enable measurement accuracy with the Leica Q Win image analysing system. Trunk size measurements were from the cemento-enamel- junction (CEJ) to a horizontal reference line through the apex of the root bifurcation parallel to the CEJ, a second measurement was from the CEJ to the alveolar margin (ALV) at the mesial, distal and bifurcation sites on the B and L of left and right M1 - 24 measurements per mandible. Results: Trunk size ranges were B 1.2-6.0 mm, L 0.6-7.8 mm; CEJ-ALV ranged between B 1.3-11.4 mm and L 0.8-7.4 mm. A General Linear Model analysis (SAS for Windows) showed statistically significant greater trunk size and CEJ-ALV measurement on the B than L at each site (P<0.0001). M1 side had no significant effect. Conclusion: Trunk size and CEJ-ALV measurements showed the same pattern as furcation rate reported in our earlier study (Volchansky, Cleaton-Jones IADR Brisbane, 2006).