The objective of this research was to determine the effect of group size on pig performance. A field study was carried out during 15 months. The building had 52 pens in total, which housed grower pigs to 35-38 kg body weight. During the first 8 months the average number of animals was 726. During the time of most significant decrease of performance in months 9 and 10 the average number went up to 751 animals. A reduction in total feed intake was observed during months 9, 10 and 11 and the average consumption of compound feed per animal and day dropped to 1.68, 1.32 and 1.09 kg. Consequently the weight gain decreased to 615, 535 and 505 g, respectively. The example showed the negative influence of overstocking of grower pigs on the essential performance indicators, such as weight gain, compound feed consumption per feeding day and feed conversion. The number of animals in pens should be decided on the basis of stocking density which influence positively performance. The size of the lying area for this particular age group and this particular building should be between 0.48 and 0.51 sq m per animal.