BackgroundPeople born in densely populated areas have a higher risk of developing schizophrenia, bipolar disorder and autism.AimsThe purpose of this study was to investigate whether urban–rural differences in place of birth influence a broad range of mental disorders.MethodPopulation-based cohort study of everyone born in Denmark between 1955 and 2006 (n = 2 894 640). Main outcome measures were incidence rate ratios for five levels of urbanisation and summary estimates contrasting birth in the capital with birth in rural areas.ResultsFor all psychiatric disorders, except intellectual disability (ICD-10 ‘mental retardation’) and behavioural and emotional disorders with onset in childhood, people born in the capital had a higher incidence than people born in rural areas.ConclusionsBirth in an urban environment is associated with an increased risk for mental illness in general and for a broad range of specific psychiatric disorders. Given this new evidence that urban–rural differences in incidence are not confined to the well-studied psychotic disorders, further work is needed to identify the underlying aetiopathogenic mechanisms.