Yes / The results of carbon and nitrogen isotope analysis from two medieval populations are presented here, in a study investigating dietary habits within a medieval hospital population in England. We used δ13C and δ15N measurements of bone collagen in order to attempt to identify a distinct group diet within the medieval hospital of St. Giles, Brough, Yorkshire, and examine the reasons why the dietary habits within the institution may have been noticeably different from that of a comparative lay population. Following the results and tentative conclusions of a study conducted by Müldner and Richards (2005), it was hypothesised that religious fasting rules would result in there being evidence of greater consumption marine fish at St. Giles than at the rural township of Box Lane, Pontefract, Yorkshire. While more dietary variation was found at the hospital, it can be seen that the differences in δ13C and δ15N isotope values vary in relation to the animal baselines. Thus, differences between the human populations can be attributed to geological and environmental factors as opposed to dietary differences.