A longitudinal study of seven litters of labrador retrievers and five litters of border collies from eight weeks to 18 months of age indicated that the majority showed some degree of potentially undesirable behaviour when separated from their owners. Its incidence was particularly high in the labrador retrievers, of which 13 of 23 showed separation-related behaviour for more than a month. Socially diverse environments experienced between six and 12 months of age were associated with a subsequent absence of separation-related behaviour. In a questionnaire survey of dog owners, separation-related behaviour was reported in 27 of 94 dogs, and a further 20 had shown the behaviour in the past. Male dogs were more likely to express separation-related behaviour currently, and females were more likely never to have displayed it. The prevalence of the behaviour was unaffected by whether the dog was pedigree or mixed breed, or whether it had been obtained from a breeder or from a rescue organisation. Combining the results of the two studies, the owners of only six of 75 dogs showing separation-related behaviour had sought assistance, and only two of the owners had sought help from veterinary surgeons.