Housing horses in open barns is becoming increasingly popular, with positive effects on social interaction and free movement that can improve horse welfare. However, many horse owners are concerned that group housing may lead to more injuries, less lying time and obesity. This thesis evaluated the effects of group housing horses an active open barn on behaviour and welfare in terms of health, rest and feeding. Horses at the Swedish National Equestrian Centre sites Strömsholm and Flyinge were studied. Health indicators such as lameness and colic were found to be lower for horses kept in the active open barn than in single boxes. Horses in the active open barn had a higher incidence of injuries due to kicks, but this did not lead to more days lost from training. Therefore, concerns about injuries in this system do not appear to be warranted. Comparisons of lying halls with different lying areas revealed that smaller lying halls led to shorter lying times compared with larger lying halls and single boxes. Greater available area in the lying hall also increased lying bouts and use of lying halls. Due to intra-individual variation, four measurements per horse were needed to establish a mean value for forage intake rate. ​​​​​​​In conclusion, keeping horses in an active open barn affects horse health and lying behaviour. Lameness and colic may decrease, probably due to free movement in the active open barn. To increase lying time among group-housed horses, the space requirement is likely to be larger, not smaller, than in individual boxes.