Research on management and performance of grower-finisher pigs has generally been conducted with small group sizes, while the swine industry has shifted towards larger group sizes. Our previous study shows that productivity of pigs was not affected by group size up to 80 pigs/pen. However, there are still concerns about aggression and injuries among pigs in a large group. The objective of this study was to determine effects of group size on aggression and injuries of pigs.