Medical errors cause significant morbidity and mortality in hospitalized patients. Specialty-based, voluntary reporting of medical errors by health care providers is an important strategy that may enhance patient safety. We developed a voluntary, anonymous, Internet-based reporting system for medical errors in neonatal intensive care, evaluated its feasibility, and identified errors that affect high-risk neonates and their families. Health professionals (n = 739) from 54 hospitals in the Vermont Oxford Network received access to a secure Internet site for anonymous reporting of errors, near-miss errors, and adverse events. Reports used free-text entry in phase 1 (17 months) and a structured form in phase 2 (10 months). The number and types of reported events and factors that contributed to the events were measured. Of 1230 reports--522 in phase 1 (17 months) and 708 in phase 2 (10 months)--the most frequent event categories were wrong medication, dose, schedule, or infusion rate (including nutritional agents and blood products; 47%); error in administration or method of using a treatment (14%); patient misidentification (11%); other system failure (9%); error or delay in diagnosis (7%); and error in the performance of an operation, procedure, or test (4%). The most frequent contributory factors were failure to follow policy or protocol (47%), inattention (27%), communications problem (22%), error in charting or documentation (13%), distraction (12%), inexperience (10%), labeling error (10%), and poor teamwork (9%). In 24 reports, family members assisted in discovery, contributed to the cause, or themselves were victims of the error. Serious patient harm was reported in 2% and minor harm in 25% of phase 2 events. Specialty-based, voluntary, anonymous Internet reporting by health care professionals identified a broad range of medical errors in neonatal intensive care and promoted multidisciplinary collaborative learning. Similar specialty-based systems have the potential to enhance patient safety in a variety of clinical settings.