This study examined the aggression that occurs when sows are regrouped. In this study the sows were grouped twice, first immediately after weaning twelve groups of four sows were mixed. This was classified as a static group, as no new sows were ever added to the group of four. Five weeks later, three static groups were mixed into a dynamic group, which contained approximately 40 sows and remained in this group for eleven weeks. In the dynamic groups, twelve sows were removed and twelve new sows were added at three-week-intervals. Upon mixing into the static pen, aggressive and submissive behaviours were recorded for the first nine hours after mixing. The social status of the sows was subsequently ranked based on their aggressiveness and the outcomes of their aggressive interactions. On the day of mixing and one week later, injuries were recorded and salivary cortisol samples were taken for both static and dynamic groups. Injuries were also recorded prior to the sows being moved into the dynamic pen. Individual sow behaviour, the order in which the sows went through the electronic feeder and where the sow rested within the pen was recorded throughout the study. The levels of sow injuries increased significantly after the sows were moved into the dynamic group from the static group. Sows with lower social status had lower bodyweights and higher levels of injuries one week after being mixed into both the static and dynamic groups. A lower social status also resulted in those sows having a lower position in the feeding ordering and they were unable to rest in the kennel area (prime lying area). Lower ranking sows were displaced from the feeder queue and drinker more often than higher-ranking sows. Social status did not have an effect on salivary cortisol levels. The results indicated that in both static and dynamic groups the welfare of the sows with a lower social rank was impaired.