Interest has focused recently on measuring severity of illness, both to improve the fairness of diagnosis related group-based reimbursement and to facilitate judgments about hospital quality of care. MedisGroups is a prominent, proprietary severity-measurement system, recently mandated for use by all Pennsylvania hospitals. We reviewed MedisGroups and its key clinical findings. MedisGroups produces admission scores, from 0 through 4, indicating increasing risk of imminent organ failure. Score computation is independent of diagnosis, but many key clinical findings are disease specific and require particular diagnostic technologies. Using a database including patients 65 years of age and older from 24 hospitals, we found that fewer than 1% of patients with scores of 0 or 1 died in-hospital, compared with 60% of those with scores of 4. Questions remain about the impact of the procedural nature of many key clinical findings and the independence from diagnosis. Further study is needed to determine the utility of MedisGroups for policy purposes.