Enthesopathies, in the guise of musculoskeletal skeletal stress markers (MSM), have been widely used to reconstruct activity levels in human skeletal populations. In general, studies have focused on their presence in the upper limb, which is used in the majority of daily activities. The aim of this study was to use some of the attachment sites on the humerus to explore the relationship between enthesopathy formation, activity, and the ageing process. The skeletal sample used in this study comprised male adult skeletons with known age-at-death and known occupations from the late-19th and early-20th century cemeteries in Portugal. The enthesopathies were recorded as either present or absent. Statistical analysis using Fishers exact tests and logistic regression was undertaken to determine whether associations could be found between specific activities or socioeconomic status (manual or nonmanual workers), and age and enthesopathy presence. Left and right sides were analyzed separately. Fisher's exact tests were used to determine the relationship between activity and enthesopathy, and they demonstrated no association between activity and enthesopathies (P > 0.01). The results of the logistic regression established that age was the single most significant factor in enthesopathy formation (P > 0.05). This study found that, in these samples, age-at-death, and therefore age-related degeneration rather than degeneration caused by activities, was the primary cause of enthesopathy formation. Considering the difficulties of reliably ageing adult human skeletal remains, this is a major issue for studies of activity using enthesopathies. Am J Phys Anthropol, 2010. © 2009 Wiley-Liss, Inc.