The financial costs associated with Adverse Events (AEs) for older patients (> or = 65 years) in Canadian hospitals are unknown. The objective of this paper is to describe and compare costs between patients who experienced an AE and those who did not during an acute hospital admission to a tertiary care facility. Patients with an AE had twice the hospital length of stay (20.2 versus 9.8 days, p < 0.00001), resulting in 1,400 extra days at a cost of approximately $7,500/patient.