Previous research demonstrates the size of secondary osteons varies considerably between individuals, though what factors act in the delineation of osteon size remain uncertain. This study explores the influence of age, sex, percent cortical area (%Ct.Ar), percent cortical porosity (%Po.Ar), and loading environment on osteon area (On.Ar) in human ribs. The sample consisted of midshaft 6(th) ribs ... [Show full abstract] from 80 individuals, 6-94 years of age. T-tests demonstrated no significant differences in On.Ar between the sexes (p=0.383). Age showed a significant correlation with both %Ct.Ar and %Po.Ar, so a hierarchical regression model was used to control for the effects of age on the other variables. Results indicate that age is the most significant factor of those tested in this study, (p=0.004), with %Ct.Ar playing a much smaller but still significant role (p=0.014), while %Po.Ar had no significant influence on On.Ar (p=0.443). Age demonstrates an inverse relationship with On.Ar, while %Ct.Ar has a direct relationship with On.Ar. Significant differences in On.Ar between the pleural and cutaneous cortices are attributed to variation in %Ct.Ar of each cortex. Therefore, age and %Ct.Ar, account for the majority of osteon size variability in this study, although it is likely genetics play an important role as well. Understanding the biological mechanisms that act in remodeling and determine osteon size is essential for accurately addressing and interpreting histological findings, work that is invaluable in its implications for bone biology. This article is protected by copyright. All rights reserved.