Objective: Persons with severe mental illness die 15-20 years earlier on average than persons without severe mental illness. Although infection is one of the leading overall causes of death, no studies have evaluated whether persons with severe mental illness have a higher mortality after infection than those without. Method: The authors studied mortality rate ratios and cumulative mortality proportions after an admission for infection for persons with severe mental illness compared with persons without severe mental illness by linking data from Danish national registries. Results: The cohort consisted of all persons hospitalized for infection during the period 1995-2011 in Denmark (N=806,835), of whom 11,343 persons had severe mental illness. Within 30 days after an infection, 1,052 (9.3%) persons with a history of severe mental illness and 58,683 (7.4%) persons without a history of severe mental illness died. Thirty-day mortality after any infection was 52% higher in persons with severe mental illness than in persons without (mortality rate ratio=1.52, 95% CI=1.43-1.61). Mortality was increased for all infections, and the mortality rate ratios ranged from 1.27 (95% CI=1.15-1.39) for persons hospitalized for sepsis to 2.61 (95% CI=1.69-4.02) for persons hospitalized for CNS infections. Depending on age, 1.7 (95% CI=1.2-2.2) to 2.9 (95% CI=2.0-3.7) more deaths were observed within 30 days after an infection per 100 persons with a history of severe mental illness compared with 100 persons without such a history. Conclusions: Persons with severe mental illness have a markedly elevated 30-day mortality after infection. Some of these excess deaths may be prevented by offering individualized and targeted interventions.