The prevalence of torus mandibularis was assessed in two groups of dental patients, altogether 2010 individuals over 10 yr of age: 1181 individuals native to the Lofoten Islands in North Norway, situated at 68 degrees latitude; and 829 patients indigenous to Gudbrandsdal, an inland district in the southeastern part of the country at 61 degrees latitude. Both groups were supposed to be of the same Caucasian stock and, therefore, to have similar genetic predisposition to torus on the average. The following observations were found: 1) the prevalence of torus mandibularis was much greater in Gudbrandsdal than in Lofoten (P much less than 0.001); 2) the prevalence decreased among persons above 50 yr of age as compared with those of the age classes 10-49 yr (P less than 0.01); and 3) it was smaller among women than men (P less than 0.05), mainly due to such a decrease in Lofoten. In a recent investigation of people living in Gudbrandsdal the fraction of the variation of torus that was attributable to genetic differences was estimated as about 30%, whereas approximately 70% of the causes seemed to be ascribable to environmental influences in terms of occlusal stress. It is suggested that dietary habits and number of existing teeth seemed to be environmental variables with an influence on the observed variation of torus prevalence between geographical regions, age classes, and sexes. The question about a possible sexual difference as to the genetic component of liability to torus mandibularis was outside the scope of the present study.