Severity of illness indexes for hospitalized patients are frequently used for the assessment of hospital performance. The purpose here was to develop and validate a quantitative index for clinical areas which could measure severity of illness during hospitalization period and would be easy to obtain from the information contained in a regular clinical chart. The construction included item selection and search for items weights. Experts and literature were consulted and 74 clinical records provided empirical information. The result was the proposal of an index with two alternatives: one quantitative and the other ordinal with four levels of severity. Validation included four aspects of validity, general reliability, interrater agreement and internal consistency. Sixteen specialized physicians assessed content and face validity. One hundred clinical records of discharged patients were used to assess criterion, construct validity and reliability. Results show satisfactory validity in almost all aspects. The reliability coefficient was 0.95, Kappa coefficient was 0.4 for the ordinal index and correlation coefficients over 0.93 for the quantitative one. The index is ready for current applications in this context although some suggestions for future improvements were also included.