In the UK, welfare organisations care for several thousand dogs each year. The successful rehoming of these dogs is a difficult process resulting in some of them being returned to rehoming centres. There have been very few studies examining the underlying mechanisms in the UK. Therefore, this study was conducted to determine those factors which affect the success of rehoming dogs. A prospective cohort study was conducted using a sample of 5750 dogs rehomed by Dogs Trust, a UK based dog welfare charity, during 2005. Dogs were followed up for a period of 6 months after adoption to determine if these dogs were still in their placement home. There was a 78% response rate to the follow-up postal questionnaires sent to the new owners, giving information on 4500 owners. Fourteen percent of adoptions failed. The results showed that behavioural problems are an important factor in the success of adoption such that if dogs had shown aggression towards people and the owners had not sought advice, they had 11.1 times the odds (95% CI: 6.6, 18.8) of being returned compared to those dogs without behavioural problems. Attending training classes significantly decreased the chance that the adoption would be unsuccessful (OR 0.3, 95% CI: 0.2, 0.4). It was shown that those owners who found that the effort and work involved in looking after their dog to be more than they had expected, had 9.9 (95% CI: 4.1, 24.6) times the odds of returning their dog than those who found the effort required to be less than they had expected. The results of this study show that there are many factors involved in a successful adoption and it is important that the new owners are informed of what to expect are encouraged to attend training classes and are prepared to work at any behavioural problems that their dog may have.