Background: Chronic diseases are common among intensive care unit (ICU) patients and may worsen their prognosis. We examined the prevalence and impact of pre-admission/index morbidity among ICU patients compared with a general population cohort. Methods: Our study encompassed all 28,172 adult patients admitted to ICUs in northern Denmark in 2005-2007 and 281,671 age- and sex-matched individuals from the general population. We used a nationwide hospital registry to obtain a 5-year history of 19 chronic diseases and computed Charlson Comorbidity Index (CCI) for each study participant and grouped them into low (CCI=0), moderate (CCI=1-2), and high (CCI=3+) morbidity levels. We computed mortality and mortality rate ratios (MRRs) adjusted for confounders, and compared the mortality between ICU patients and the general population cohort. Results: Low, moderate, and high pre-admission morbidity levels were present in 51.5%, 34.1%, and 14.4% of ICU patients, respectively. In these groups, 30-day mortality was 10.8%, 18.4%, and 26.7%, respectively. Three-year mortality was 21.3%, 43.1%, and 63.2%, respectively. The adjusted 30-day MRR was 1.30 [95% confidence intervals (CI): 1.21-1.39] and 1.86 (95% CI: 1.71-2.01) for ICU patients with moderate and high morbidity levels, both compared with a low morbidity level. The general population had a lower morbidity level and mortality at all morbidity levels throughout the study period. Interaction between ICU admission and high morbidity level added 5.1% to the mortality during the second and third year of follow-up. Conclusion: A high pre-admission morbidity level was frequent among ICU patients and associated with a worsened prognosis. Morbidity had more impact on mortality among ICU patients compared with a general population cohort.