Animal welfare is an important aspect of organic broiler (OB) production which involves e.g. up to twice the rearing period compared with conventional production and feedstuffs without prophylactic additives or synthetic amino acids. As market demand for organic broiler meat increases, the formerly modest OB production in Sweden is now rapidly increasing and research is needed to support this development. It is essential to use genotypes that adapt well to the rearing environment, in order to achieve satisfactory animal welfare. Health problems related to fast growth rate are a common animal welfare issue in modern broilers and this applies to systems with long rearing periods. In this study, the Welfare Quality® assessment protocol for poultry was used for assessment of lameness, contact dermatitis, cleanliness, thermal comfort, litter quality and the human-animal relationship in two modern broiler genotypes; the fast-growing Ross 308 (R) and the slower-growing Rowan Ranger (RR). In total, 645 day-old chicks (328 R and 317 RR) were reared until 10 weeks of age, which is the rearing period formerly used in organic production in Sweden, and fed a low-protein (L), high-protein (H) or mussel-meal (M) diet containing 14.5%, 17.0% or 15.6% crude protein, respectively. Broiler welfare was assessed on three occasions (at weeks 2, 6 and 9). The results showed rapid deterioration in welfare for fast-growing broilers when kept beyond 6 weeks. Mortality rate and incidence of lameness and contact dermatitis increased and litter quality, thermal comfort and plumage cleanliness decreased. Indications of poor welfare were also observed in the slower-growing hybrid, but to a lesser extent and later during rearing. Diet type only had minor effects on bird welfare, although R birds grew faster on the M diet. Thus the slower-growing RR hybrid is preferable to the fast-growing broiler type in production systems with a long rearing period. However, the RR growth rate can be regarded as moderate and, to avoid health problems related to fast growth rate, hybrids that grow even more slowly should be considered for OB production systems with a long rearing period.