Medical error is an inaccurate or incomplete diagnosis and/or treatment of a disease; injury; syndrome; behavior; infection or other ailment.

In the U.S., medical errors are estimated to result in 44,000 to 98,000 unnecessary deaths and 1,000,000 excess injuries each year. One older extrapolation suggests '180,000 people die each year partly as a result of iatrogenic injury, the equivalent of three jumbo-jet crashes every 2 days'. It is estimated that in a typical 100 to 300 bed hospital in the United States, excess costs of $1,000,000 to $3,000,000 attributable to prolonged stays and complications just due to medication errors occur yearly.

However, medical error definitions are subject to debate, as there are many types of medical error from minor to major, and causality is often poorly determined. The Health Grades study statistics, based on AHRQ MedPAR data, were based on administrative records, not clinical records, and largely overlooked multi-causality of outcomes.

Medical care is frequently compared adversely to aviation: while many of the factors which lead to errors in both fields are similar, aviation's error management protocols are regarded as much more effective.

Experts estimate that as many as 98,000 people die in any given year from medical errors that occur in hospitals. That's more than die from motor vehicle accidents, breast cancer, or AIDS--three causes that receive far more public attention. Indeed, more people die annually from medication errors than from workplace injuries. Add the financial cost to the human tragedy, and medical error easily rises to the top ranks of urgent, widespread public problems.

In 2000, The Institute of Medicine released "To Err Is Human", which asserts that the problem in medical errors is not bad people in health care--it is that good people are working in bad systems that need to be made safer.