This article evaluates the behavior of an adaptation of the Charlson Index (CHI) applied to administrative databases to measure the relationship between chronic comorbidity and the hospital care outcomes of length of stay (LOS), in-hospital mortality, and emergency readmissions at 30 and 365 days. These outcomes were analyzed in 106,673 hospitalization episodes whose records are registered in a minimum basic data set maintained by the public health authorities of the community of Valencia, Spain. The highest comorbidity measured by the CHI was associated with greater LOS and in-hospital mortality and increased readmission at 30 and 365 days. The rate of readmissions at 1 year dropped, however, in the group with the greatest comorbidity, probably owing to an increase in mortality after hospitalization. While comorbidity does appear to increase the risk of adverse outcomes in general and mortality and readmission specifically, the second outcome is only possible if the first has not occurred. For this reason, information and selection biases derived from administrative databases, or from the CHI itself, should be taken into account when using and interpreting the index.