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A recent telephone survey of 735 Dutch farmers with group housing of sows in early pregnancy suggested a wide variation in reproductive success, irrespective of husbandry system. The reason for these differences between farms is not known, but of great importance to the pig industry which is moving worldwide from individual to group housing systems, mainly for animal welfare reasons. Therefore, the aim of this review is to list the most likely success and risk factors for group housing of sows in early pregnancy, in relation to reproduction (such as pregnancy rate) and animal welfare (such as aggression and health). The review first considers a limited number of group housing comparisons and finds that the results of the few studies performed are not very conclusive, neither on fertility nor on leg problems, longevity and welfare of sows. As a consequence, it is hard to draw any significant practical conclusions, and the review proceeds to look at studies in which single factors were addressed. For the effects on fertility it is concluded that especially factors causing chronic stress (e.g. due to unfavourable social, management or climatic conditions) and a low feed intake may impact reproductive performance. The vulnerable period lies in weeks 2 and 3 of pregnancy. On leg problems and longevity an important factor seems to be the quality of the floor, regardless of other aspects of the housing system. The use of straw seems to be advantageous. Finally, on aggression it is argued that this will never be eliminated from systems in which sows are housed as a group. However, it can be properly managed and key factors in this are a gradual familiarisation of unfamiliar animals, sufficient space and pen structure during initial mixing, minimising opportunities for dominant sows to steal food from subordinates, the provision of a good quality floor and the use of straw bedding. The review concludes with a series of practical recommendations to improve welfare and reprodu