This study purports to show the relationship between volume and patient harm due to health care error. Using 5 measures of volume and incident reports weighted for patient harm over the course of 515 days, it is shown that increased volume is related to increased harm to patients. As the number of areas in the hospital experiencing high volume increased, the likelihood of patients sustaining serious harm because of health care error also increased. This is attributed to reaching system capacity causing support services (ie, lab, pharmacy, radiology, housekeeping and engineering) to be overwhelmed and unable to keep up with requests from caregivers.