The validity of administrative data may be vulnerable to how well physicians document medical charts. The objective of this study is to determine the relationship between chart documentation quality and the validity of administrative data. The charts for patients who underwent carotid endarterectomy were re-abstracted and rated for the quality of documentation. Poorly and well-documented charts were compared by patient, physician, and hospital variables, as well as on agreement between the administrative and re-abstracted data. Of the 2061 charts reviewed, 42.6 per cent were rated well documented. The proportion of charts well documented varied from 14.6 to 87.5 per cent across 17 hospitals, but did not vary significantly by patient characteristics. The kappa statistic was generally higher for well-documented charts than for poorly documented charts, but varied across comorbidities. In conclusion, poorly documented hospital charts tend to be translated into invalid administrative data, which reduces the communication of clinical information among healthcare providers.