BACKGROUND: Living arrangements have changed markedly in recent decades, so we wanted to provide an up-to-date assessment of mortality as a function of marital status and cohabitation status in a complete population. METHODS: We studied mortality in a national cohort of 6.5 million Danes followed for 122.5 million person-years during 1982-2011, using continuously updated individual-level information on living arrangements, socio-demographic covariates and causes of deaths. Hazard ratios (HRs) estimated relative mortality in categories of marital status, cohabitation status and combinations thereof. RESULTS: HRs for overall mortality changed markedly over time, most notably for persons in same-sex marriage. In 2000-2011, opposite-sex married persons (reference, HR = 1) had consistently lower mortality than persons in other marital status categories in women (HRs 1.37-1.89) and men (HRs 1.37-1.66). Mortality was particularly high for same-sex married women (HR = 1.89), notably from suicide (HR = 6.40) and cancer (HR = 1.62), whereas rates for same-sex married men (HR = 1.38) were equal to or lower than those for unmarried, divorced and widowed men. Prior marriages (whether opposite-sex or same-sex) were associated with increased mortality in both women and men (HR = 1.16-1.45 per additional prior marriage). CONCLUSION: Our study provides a detailed account of living arrangements and their associations with mortality over three decades, thus yielding accurate and statistically powerful analyses of public health relevance to countries with marriage and cohabitation patterns comparable to Denmark's. Of note, mortality among same-sex married men has declined markedly since the mid-1990s and is now at or below that of unmarried, divorced and widowed men, whereas same-sex married women emerge as the group of women with highest and, in recent years, even further increasing mortality.