Frequencies of maxillary sinusitis in the population of the medieval town Sigtuna, Sweden, were examined. The first aim was to explore the occurrence of sinus conditions in this urban population, and the second was to investigate the effects of preservation in relation to these changes. The skeletal sample consisted of 274 adolescent and adult individuals buried ca 970–1530 AD. The maxillary sinuses were examined for different types of skeletal change related to sinusitis. The severity and location of these changes were recorded, together with the preservation of the sinus. When the diagnostic criteria for bone preservation from previous studies was used, i.e. the preservation of at least one complete antral floor, the results showed that 97.7% (n = 157) of the individuals were affected, with older individuals showing more severe bone changes. In the entire (i.e. more fragmented) sample, 94.5% (n = 259) showed signs of sinusitis. Again, older individuals showed more severe bone changes, and males were more frequently affected. Also, an increased frequency over time was found. The maxillae were subdivided into groups of bone preservation, and the results showed that there was a significant relation between preservation and the registered presence and severity of sinusitis. The results may be used as a cautionary note against investigating the occurrence of maxillary sinusitis without considering the skeletal preservation factor. There was no correlation between preservation and sex or age groups. This suggests that the differences between younger and older and male and female were not a result of preservation alone and could be considered as trustworthy. Copyright © 2011 John Wiley & Sons, Ltd.