Increasing the group size of weaned pigs in commercial systems has potential benefits in terms of leading to more efficient use of resources and greater ease of management. There are concerns, however, about the effects of increasing group size on productivity and welfare. Another concern in commercial systems is the effect that group size during the post-weaning period has on performance during the finishing period. In some production systems, pigs which are housed in large groups during the post-weaning period may have to be split into smaller groups during the finishing period in order to suit finishing accommodation. This regrouping may lead to increased stress and may in turn lead to reduced productivity. The aim of the present study was to assess the effect of increasing group size up to 60 pigs per group on performance and behaviour during the post-weaning period (4 weeks of age until 10 weeks). In addition, the effects on performance of splitting groups of 40 and 60 pigs into groups of 20 pigs at the start of finishing period (11 weeks of age until 21 weeks of age) was tested. Group size did not significantly affect growth rate and feed intake, however food conversion tended to be poorer in groups of 40 and 60 pigs than in smaller group sizes. Behavioural observations did not show any difference among the group sizes in the proportion of small, medium and large sized pigs at the feeder, or in the total number of pigs at the feeder. In addition the overall aggression at the feeder was similar in all group sizes. Food conversion ratio was poorer in groups split from 60 pigs than in groups of 40 pigs, or in groups which were kept in group size of 20 pigs throughout the growing and finishing stages. The results suggest that the group size of weaned pigs can be increased from 10 to 60 animals without any significant adverse effects on performance or welfare during the post-weaning period. If the pigs are to be housed in groups of 20 during the finishing period then it may be better to house pigs in groups of 40 rather than in groups of 60 pigs during the post-weaning period, in order to reduce negative effects on food conversion during the finishing period.