This study examined the prevalence of behaviour problems exhibited by dogs within 4 weeks of acquisition from a rescue shelter in Northern Ireland. One thousand five hundred and forty-seven people who had purchased a dog from a rescue shelter in Northern Ireland were sent a postal questionnaire designed to collect information on the behaviours exhibited by their dog within the first month of acquisition. Five hundred and fifty-six people responded to the survey, representing a response rate of 37%. The majority of respondents (68.3%) reported that their dog exhibited a behaviour problem, the most common being fearfulness. Most of those respondents (89.7%) who returned their dog to the shelter did so because the animal exhibited behaviour that they considered undesirable. Male dogs showed more unacceptable behaviours than females, specifically inter-male aggression, sexual problems and straying tendencies. More stray dogs displayed undesirable behaviour than unwanteds, specifically straying tendencies. Puppies were less likely to exhibit unacceptable behaviours than juveniles or adults, particularly fearfulness, sexual problems and straying tendencies. More juvenile dogs showed excessive activity and excessive barking than puppies or adults. More adult dogs displayed aggression towards other dogs than juveniles or puppies. Findings indicate that dogs purchased from rescue shelters do exhibit behaviour problems that may lead to their return. The number of dogs admitted or returned to rescue shelters with behaviour problems may be reduced by raising public awareness regarding the value of behaviour therapy and introducing behaviour therapy schemes to rescue shelters.