Clinicians, departments and trusts are compared using Hospital Standardised Mortality Ratios. The benchmarking process makes corrections for patients' past medical histories, using a tool called the Charlson Index of Co-morbidity. Patients with any of 17 conditions receive an adjustment to their expected mortality and length of stay. Although the weighting of these conditions has been altered to take account of advances in medical practice, there have been no conditions added to the list since Charlson's original paper of 1986. The Charlson Index of Co-morbidity does not make a correction for Parkinson's disease. We describe evidence of the impact of Parkinson's disease as a co-morbid condition and recommend that, in future, benchmarking processes should take account of this important and common diagnosis.