Large group sizes (50 – 2000 pigs) are an increasingly common feature of pig production. It has been associated with greater efficiency of building space use, improved mechanization and reduced labour input per pig. However the economic and animal welfare disadvantages have not been adequately addressed. In this review the implications of group size (range 3 – 120 pigs) for growth performance are quantified using regression analysis of data from 20 earlier studies and almost 22,000 animals. The relationships between group size and immunocompetence, clinical health status, within-group variation in growth rate, carcass characteristics, the occurrences of vices and reproductive performance are reviewed in this paper. A significant, negative, approximately linear relationship in average daily gain (ADG) with increases in group size was recorded during the weaner (weaning to 30 kg) and grower (31-68 kg) stages. Weaner stage pigs showed a reduction in food intake that was not present during the grower stage. Consequently the efficiency of growth was compromised during the grower stage. No influence of group size on performance during the finisher stage (>69 kg) was detected. None of the other traits measured were consistently and significantly influenced by changes in group size. A large group size may compromise the growth performance of young pigs, but the long-term consequences for other economically important traits is like to be slight.