Yes / In palaeopathology, a well-established approach to malnutrition and ill-health is the study of metabolic conditions. Leprosy is a mycobacterial disease that is manifested on the bones, and is commonly studied in archaeological contexts. Vitamin D is essential for maintaining a normal immune system, and thus a metabolic insufficiency could have a major effect in the resistance of an individual to invading pathogens. It has been indicated by clinical studies that there is an increase in the risk of contracting tuberculosis for individuals with Vitamin D deficiency, and like TB, leprosy is a disease of the poor, and it is more severe in individuals with low resistance to the pathogen. The project investigated the immunological aspect of leprosy by investigating the comorbidity of Vitamin D deficiency and the disease. During the study, the prevalence rates of Vitamin D deficiency (residual rickets and osteomalacia) were compared for adults in two medieval populations: adults with skeletal evidence of lepromatous leprosy from the leprosarium of St James and Mary Magdalene in Chichester (n=62) and adults from the non-leprous population found in Box Lane, Pontefract (n=52), both in England. Macroscopic analysis identified only one probable case of residual rickets and two possible cases of osteomalacia, providing no statistical significance in the relationship between the conditions. The present article focuses on these results, aiming to underline the reasons behind negative results in research, caused either by failed methodology or the insufficient collection of samples.