We investigated whether replacing the Acute Physiology and Chronic Health Evaluation (APACHE) II weighted comorbidity score with other measures of prior comorbidity would improve the prediction of hospital mortality in critically ill patients. Clinical data of 24 303 critically ill patients were linked to the Western Australian hospital morbidity database to identify prior comorbidities. Minor comorbidities as described in the Charlson comorbidity index and Elixhauser comorbidities were prevalent in critically ill patients. Among 24 303 admissions, 3615 (14.9%), 10 223 (42.1%), and 11 597 (47.7%) patients had at least one comorbidity as defined in the APACHE II score, Charlson comorbidity index, and Elixhauser comorbidities, respectively. The ability of comorbidity alone to discriminate between hospital survivors and non-survivors was poor. Replacing the APACHE II weighted comorbidity score with other more comprehensive measures of comorbidity did not significantly improve the discrimination of the APACHE II score.