SummaryBackgroundMajor depressive disorder (MDD) is a common psychiatric disorder associated with a high disease burden. This study gives a comprehensive overview of the prevalence, outcomes, treatment, and genetic epidemiology of MDD within and across the Scandinavian countries.MethodsThis study has aimed to assess and compare across Norway, Denmark, and Sweden 1) the prevalence and trajectories of MDD and comorbidity, 2) outcomes and treatment, and 3) heritability (Denmark and Sweden only). The analyses leveraged data on 272,944 MDD cases (and 6.2 million non-cases) from Norway, Sweden, and Denmark in specialist care in national longitudinal health registers covering 1975–2013. Relying on harmonized public data global comparisons of socioeconomic and health metrics were performed to assess to what extent findings are generalizable.FindingsMDD ranked among the most prevalent psychiatric disorders. For many cases, the disorder trajectory was severe, with varying proportions experiencing recurrence, developing comorbid disorders, requiring inpatient treatment, or dying of suicide. Important country differences in specialist care prevalence and treatment were observed. Heritability estimates were moderate (35–48%). In terms of socioeconomic and health indices, the Scandinavian nations were comparable to one another and grouped with other Western nations.InterpretationThe Scandinavian countries were similar with regards to MDD epidemiological measures, but we show that differences in health care organization need to be taken into consideration when comparing countries. This study demonstrates the utility of using comprehensive population-wide registry data, outlining possibilities for other applications. The findings will be of use to policy makers for developing better prevention and intervention strategies.Funding10.13039/501100004359Swedish Research Council(Vetenskapsrådet, award D0886501 to PFS),10.13039/100000002US National Institutes of Mental HealthR01 MH123724 (to PFS),European Union’s Horizon 2020 Research and Innovation Program(847776 and 964874, to OA) and10.13039/501100000781European Research Councilgrant (grant agreement ID 101042183, to YL).