Objective To test the hypothesis of 'horizontal epidemiology',i.e. that psychosocial difficulties (PSDs),such as sleep disturbances, emotional instability and difficulties in personal interactions, and their environmental determinants are experienced in common across neurological and psychiatric disorders, together called brain disorders. Study Design A multi-method study involving systematic literature reviews, content analysis of patient-reported outcomes and outcome instruments, clinical input and a qualitative study was carried out to generate a pool of PSD and environmental determinants relevant for nine different brain disorders, namely epilepsy, migraine, multiple sclerosis, Parkinson's disease, stroke, dementia, depression, schizophrenia and substance dependency. Information from these sources was harmonized and compiled, and after feedback from external experts, a data collection protocol including PSD and determinants common across these nine disorders was developed. This protocol was implemented as an interview in a cross-sectional study including a convenience sample of persons with one of the nine brain disorders. PSDs endorsed by at least 25% of patients with a brain disorder were considered associated with the disorder. PSD were considered common across disorders if associated to 5 out of the 9 brain disorders and if among the 5 both neurological and psychiatric conditions were represented. Setting The data collection protocol with 64 PSDs and 20 determinants was used to collect data from a convenience sample of 722 persons in four specialized health care facilities in Europe. Results 57 of the PSDs and 16 of the determinants included in the protocol were found to be experienced across brain disorders. Conclusion This is the first evidence that supports the hypothesis of horizontal epidemiology in brain disorders. This result challenges the brain disorder-specific or vertical approach in which clinical and epidemiological research about psychosocial difficulties experienced in daily life is commonly carried in neurology and psychiatry and the way in which the corresponding health care delivery is practiced in many countries of the world.