Medication errors are pervasive in America's health care system. MEDMARX is an Internet-accessible, anonymous medication error reporting program designed for hospitals and health systems to systematically collect, analyze, and report medication errors. This study examined 154,816 medication error reports that were submitted to MEDMARX between January 1, 1999, and December 31, 2001. Data fields analyzed were error category (based on the National Coordinating Council for Medication Error Reporting and Prevention's Error Category Index), type(s) of error, cause(s) of error, contributing factor(s), and product(s) involved. Approximately two-thirds of the errors reported reached the patient, with relatively few causing harm. Death was reported in 19 occurrences. Errors of omission and improper dose/quantity were the most commonly reported. Performance deficit and procedure/protocol not followed were consistently identified as causes of error. Distractions and workload increase were often cited as contributing factors. There was a similar pattern of products reported in each of the years. Implications for quality assurance, clinical practice, and health policy are presented.