1. Medication errors should be amenable to epidemiological analysis, giving insights into the causes of error and the effects of interventions to prevent them or reduce harm from them. 2. There are formidable difficulties in establishing the rates of medication errors. 3. There is no agreement on a clear operational definition of the condition. 4. The methods used to enumerate cases so far have been unreliable or incomplete or both. 5. There is disagreement about whether cases of error that do not cause harm should be included in calculations of error rates. 6. When harm occurs in association with drug therapy, it is often unclear whether the harm might have been prevented, and its occurrence should therefore be considered to result from error. 7. The denominator for calculating the rate of error is both ill-defined and inconsistently measured. Better definitions, more complete evaluation, and more thorough impact assessment may improve matters.