Comorbidities are conditions that coexist with a disease of interest, and may lead to a delayed diagnosis, be confounders in analysis of clinical status and course, and increase morbidity and mortality. Therefore, it appears desirable to summarise efficiently one or multiple comorbidities into a single score in an efficient manner, using comorbidity indices and self-administered comorbidity questionnaires. The two most commonly used comorbidity indices are the Charlson Comorbidity Index (CCI) and the Elixhauser et al. comorbidity measure (ECM). The CCI was constructed based on the mortality rates of 607 patients admitted to the general internal medicine service over 1 month; sixteen diseases were included in this index, with different weights, and were selected and weighted based on the strength of their association with mortality. Elixhauser et al. used administrative data to identify the 30 comorbidities that had a major impact on short-term outcomes in acutely hospitalised patients. Although ECM appeared to have better performance in all aspects of validity, difficulty in terms of feasibility in collecting 30 comorbidities may encourage investigators to use the CCI. Self-administered questionnaires could be a valid and reliable alternative approach to assess comorbidities, and a tool to be included in prospective studies.