A proper knowledge of risks factors related to aggression may help to develop preventive measures. One of the aspects that increase the risk of living together with an aggressive dog is the impulsiveness of the attacks. Previous studies conducted by the Animal Behaviour Service - Barcelona School of Veterinary Medicine (ABS) concluded that English Cocker Spaniel (ECS) showed a higher percentage of owner directed aggression and a higher rate of impulsiveness (lack of warning signals before an attack) than aggressive dogs of other breeds. The objective of this study was to gather further information on impulsive aggressive dogs to understand the factors that underlie impulsiveness as a risk factor. One hundred nineteen ECS presented for owner directed aggression at the ABS were evaluated. The clinical history obtained through a questionnaire to the owner provided general information and a detailed description of the aggressive episodes. The study group was compared with 155 dogs of other breeds that showed owner directed aggression. All comparisons were done through chi-square tests using the SPSS 15.0 for windows. The ECS group showed owner directed aggression with impulsiveness more frequently than dogs of other breeds (43.7% and 31 % respectively; ?2 = 4.7; p<0.03). Impulsive ECS were purchased more frequently from a pet shop than non impulsive ECS (58.3% and 38.6 % respectively; ?2 = 4.06; p<0.05). No differences were found between non impulsive ECS and non impulsive dogs of other breeds in their origin. Regarding the context in which aggression occurred, impulsive ECSs reacted more frequently when being petted than non impulsive ECSs (48.1% and 27 % respectively; ?2 = 5.7; p<0.02). In relation to other factors such as the age of adoption, the age of the dog, sex, neutering status, posture of non impulsive dogs and coat colour, no differences were found between groups.