Using methodology originally developed for the Harvard Medical Practice Study, the authors reviewed more than 4000 hospitalizations from 20 Canadian hospitals in order to estimate the degree of iatrogenic harm suffered by patients.  Overall, 7.5% of patients suffered at least one adverse event, of which more than one-third were felt to be preventable.  These data are similar to prior studies conducted in the United States and Australia, but (as with prior studies) reviewer agreement on the presence and preventability of errors was only moderate.