Abstract : It is difficult to compare the severity of different illnesses. Medical outcome measurements -- morbidity and mortality -- are not useful indicators. Another indicator is needed. A Severity of Illness Scale has been developed and validated on patients hospitalized with abdominal pain. A maximum likelihood factor analysis was carried out on the following variables: the number of bed days occupied by the patient, the number of diagnoses, procedures, preoperative, postoperative, and convalescent days, and the hospital admissions. This produced a two factor solution which reflected (i) the intensity of the treatment and (ii) the severity of the illness. There was a significant effect of the specific diagnosis on the treatment instituted, which demonstrated the validity of the Severity of Illness Scale. Other uses of the scale are discussed. Keywords: Quality of care; Abdominal pain; Submarine personnel; Technology assessment; Outcomes.