Aggression is a rising issue in many dog breeds and is an obstacle in proper dog-human interactions. However, the aggressive behaviour can also disturb the welfare of the dog. Aggression is particularly common in English Cocker Spaniels. Thus, the aim of this study was to examine factor associated with aggression of English Cocker Spaniels using an owner-completed survey. Information on aggressive behaviour was collected from 503 dogs by electronic surveys distributed and collected in November 2022 in Poland. We investigated whether the occurrence of aggression towards people and animals was affected by age, sex, adoption from a shelter, contact with children and other animals, whether the dog has a pedigree or by other behavioural disorders (jumping on people, pulling on leash, digging holes, destroying objects, disobedience, vagrancy, inappropriate urination and defecation, begging for food). We have showed that the dogs aggressive towards other animals are significantly more likely to be aggressive towards humans. Also the age of a dog, a pedigree and inappropriate urination and defecation, significantly affect which type of the animals the dog will be aggressive towards. The logistic regression was used to test the effect of different factors on aggression towards human and animals. Our results showed that aggressive behaviour toward humans is particularly exhibited by males and it increased with age. Also, dogs that do not have an official pedigree showed aggression much more frequently toward people and other animals. In addition, dogs adopted from a shelter were often more aggressive towards people. It was also found that dogs that jump on people are significantly more likely to be aggressive toward a human. Other behavioural disorders had no effect on the occurrence of aggression. Analysis of collected data provides knowledge on the factors that affect the occurrence of undesirable behaviour in English Cocker Spaniel dogs. Understanding the causes of aggression can significantly improve the relationship between owner and dog. Our findings should not necessarily preclude people from wanting to own an English Cocker Spaniels breed, rather to be prepared for a need of a proper training.