This scientific opinion describes the influence of genetic parameters that have affected the welfare of commercial broilers. There is a lack of robust scientific data for Europe on welfare outcome indicators and these should be recorded independently and made publicly available. The major welfare concerns that have a genetic basis and that may interact with management factors to lead to poor welfare include skeletal disorders, contact dermatitis, ascites and sudden death syndrome. Most of these are linked with fast growth rates. There are also numerous interactions between the environment and the genetic traits that can seriously adversely affect welfare in areas such as lighting regimes, litter management, dietary deficiencies and contamination, air quality and temperature. In the risk assessment the probability of exposure to a hazard, and the magnitude of the poor welfare effects (consequences) of that exposure were estimated. The major risk scores were unbalanced body conformation, high stocking density, fast growth rate, low light intensity and wet litter. The top ranking environmental hazards were high stocking density, low light intensity and wet litter. It was recognised by the experts that probabilities vary from region to region, country to country and among different types of farming system. Recommendations include better data collection in Europe, greater selection strategies for improved welfare traits with birds being selected and tested for their subsequent rearing and production environments by the breeders. Finally, a high priority should be given to decreasing the proportion of birds with the higher gait scores, and to include contact dermatitis and other welfare traits in the selection schemes.