A study was conducted to investigate the clinical aspects and to define the mode of inheritance of idiopathic epilepsy in the Bernese mountain dog. Pedigree analyses were carried out on an open, non-preselected population of 4005 dogs. Five different subpopulations with 50 epileptic dogs from 13 generations were included. Almost ail epileptic patients showed generalised seizures of the grand-mal type with a well-defined prodromal and postictal phase. The majority (62 per cent) of the epileptic dogs had had their first seizures at between one and three years of age and it was found that the age at first seizure was significantly (P<0·05) lower in dogs from affected parental animals than in dogs from healthy parental animals. A clear predisposition for males was also noted. Additionally, there was no correlation between inbreeding coefficient and age at first seizure or incidence rate of seizures. The increased occurrence of the disease in different subpopulations and different families of the same sires or dams showed that there was a genetic basis for the condition in the Bernese mountain dog. Furthermore, the results of the pedigree analyses and binomial test support the hypothesis that idiopathic epilepsy has a polygenic, recessive mode of inheritance in the breed. Additional objective test-mating programmes would however be necessary to define the exact mode of inheritance.