PurposeThe aim of this study is to assess the effect of comorbidities on risk of readmission to an intensive care unit (ICU) and the excess hospital mortality associated with ICU readmissions.Materials and MethodsA cohort study used clinical data from a 22-bed multidisciplinary ICU in a university hospital and comorbidity data from the Western Australian hospital morbidity database.ResultsFrom 16 926 consecutive ICU admissions between 1987 and 2002, and 654 (3.9%) of these patients were readmitted to ICU readmissions within the same hospitalization. Patients with readmission were older, more likely to be originally admitted from the operating theatre or hospital ward, had a higher Acute Physiology and Chronic Health Evaluation (APACHE)-predicted mortality, and had more comorbidities when compared with patients without readmission. The number of Charlson comorbidities was significantly associated with late readmission (>72 hours) but not early readmission (≤72 hours) in the multivariate analysis. Both early and late ICU readmissions were associated with an increased risk of hospital mortality (odds ratio, 1.68; 95% confidence interval, 1.18-2.39; P = .004; odds ratio, 1.45; 95% confidence interval, 1.05-1.99; P = .022, respectively) after adjusting for age, admission source, type of admission, the APACHE-predicted mortality, and the number of Charlson comorbidities and APACHE chronic health conditions.ConclusionsComorbidity was a risk factor for late ICU readmission. Comorbidities could not account for the excess mortality associated with ICU readmissions.