A total of 3033 skeletons from 11 medieval Danish cemeteries and 99 skeletons from the North Scandinavian medieval site of Westerhus were examined for seven lesions indicative of leprosy. The seven lesions are: rounding to the edge of the nasal aperture, degeneration of spina nasalis anterior, degeneration of the alveolar process of the pre-maxilla, porosity or perforation of the palatine process of maxilla, sub-periostal exostoses on the fibula, general swelling of the shaft of the fibula, and degeneration of the 5th metatarsal bone. The dichotomous scores of these lesions were used to estimate sensitivity and specificity of the lesion scores in relation to leprosy and to estimate sample point prevalence of leprosy at death among adults. In turn the estimates of sensitivity and specificity were used to calculate an individual comprehensive statistic, lamda, indicating leprosy status. Among adults the lamda statistic did not associate with age at death, but this cannot be taken as a sign of lack of selective mortality for leprosy but a combination of the opposing effects of long waiting time before developing leprosy related lesions and short survival with these lesions. In urban communities sufferers of leprosy were institutionalized when the leprosarium was established (in Odense around 1275); in rural communities this did not happen but the pattern of burial does indicate an internal segregation of sufferers. In the early Middle Ages (AD 1150-1350) the point prevalence at death among adults of leprosy was higher in rural (25-40 percent) than in urban (10-20 percent) communities, and villages close to town showed lower frequencies of leprosy than villages situated further away from these centers. Leprosy declined in the late Middle Ages, first in towns and cities, later in rural communities. In Odense and Malmö it appears that leprosy was effectively eliminated by 1350 whereas there were still sufferers of leprosy at Øm Kloster around 1550. Leprosy appears to have been less common in North Scandinavia than in South Scandinavia, and there are some indications that leprosy was much more common in the Sámi population than in the North population of North Scandinavia. It is suggested that the rapid and early decline of leprosy in the towns was caused by the breaking of chains of infection by institutionalizing the most affected sufferers from leprosy. In rural communities it is suggested that the later decline of leprosy was brought about by a natural vaccination with the active substance in the Calmette vaccine, Mycobacterium bovis.