The objective was to compare the characteristics of medication errors reported to 2 national error reporting systems by conducting a cross-sectional analysis of errors reported from adult intensive care units to the UK National Reporting and Learning System and the US MedMarx system. Outcome measures were error types, severity of patient harm, stage of medication process, and involved medications. The authors analyzed 2837 UK error reports and 56 368 US reports. Differences were observed between UK and US errors for wrong dose (44% vs 29%), omitted dose (8.6% vs 27%), and stage of medication process (prescribing: 14% vs 49%; administration: 71% vs 42%). Moderate/severe harm or death was reported in 4.9% of UK versus 3.4% of US errors. Gentamicin was cited in 7.4% of the UK versus 0.7% of the US reports (odds ratio = 9.25). There were differences in the types of errors reported and the medications most often involved. These differences warrant further examination.