Occurrence of behavioural disorders in domestic cats. 269 cat owners was aimed at collecting information on abnormal/undesired behaviours of their pets. Based on this information, influence of sex, breed, age, origin, age at purchase/adoption and living conditions (number of cats, presence of other animals and/or children) on respective behaviours was studied. Results were analyzed with Chi² test and V-Kramer coefficient was calculated to establish level of correlations between different features. Abnormal/undesirable behaviours were reported in 73% of cats. 50% of them would scratch walls and furniture, and this behaviour was influenced by age at purchase/adoption, absence of other animals, and age of children in the family. 28.6% of cats showed urination/defecation in random places and this was influenced by age (more common in older animals), reproductive status (more frequent in neutered animals, irrespective of their sex) and presence of other animals. Aggression towards other cats occurred in 20% of animals, most of them over 8 years of age and it was also far more common in cats allowed to spent at least some time outdoor. Aggression towards animals of other species was found in 10% of cats, significantly more often in females and cats of both sexes roaming free. Other abnormal//undesirable behaviours were: destruction (8%), aggression towards people (significantly more frequent in households with children), excessive grooming (2.6%) and autoaggression (1.9%).