BACKGROUND: Assortative mating has been demonstrated in mental disorders but the extent of cohabitation between patients with clinically diagnosed psychiatric disease has been poorly explored. Method We conducted a register-based study of all Danes between 18 and 70 years of age in a 13-year observational period, linking data on individuals' contacts with psychiatric services with data on individuals' cohabitation status. Two different Poisson regression analyses were performed: the first comparing the rates of commencing cohabitation with a psychiatric patient between individuals, depending on whether the individuals themselves had, or did not have, a psychiatric diagnosis; the second comparing the incidence rates of psychiatric diagnoses for individuals cohabitating with psychiatric patients with the similar rates for individuals living with unaffected cohabitants. RESULTS: In total, 159 929 (5.0%) out of 3 204 633 individuals were given a psychiatric diagnosis during the study period. Diagnosed individuals had an overall rate ratio (RR) of commencing cohabitation with a psychiatric patient of 1.95 [95% confidence interval (CI) 1.90-2.00] for women and 1.65 (95% CI 1.61-1.69) for men, when compared with unaffected individuals. The overall RR of receiving a psychiatric diagnosis while cohabitating with a psychiatric patient was 2.40 (95% CI 2.31-2.49) for women and 2.91 (95% CI 2.81-3.01) for men, when compared with those cohabitating with unaffected individuals. Individuals with schizophrenia and men with bipolar disorder had the highest RR of commencing cohabitation with a cohabitant with a similar diagnosis. CONCLUSIONS: Cohabitation among individuals with severe psychiatric disorders is increased. This has implications for research and for the clinical management of patients.