BACKGROUND: Hospital accreditation and International Standardisation Organisation (ISO) certification offer alternative mechanisms for improving safety and quality, or as a mark of achievement. There is little published evidence on their relative merits. OBJECTIVE: To identify systematic differences in quality management between hospitals that were accredited, or certificated, or neither. Research design ANALYSIS: of compliance with measures of quality in 89 hospitals in six countries, as assessed by external auditors using a standardized tool, as part of the EC-funded METHODS: of Assessing Response to Quality Improvement Strategies project. MAIN OUTCOME MEASURES: Compliance scores in six dimensions of each hospital-grouped according to the achievement of accreditation, certification or neither. RESULTS: Of the 89 hospitals selected for external audit, 34 were accredited (without ISO certification), 10 were certificated under ISO 9001 (without accreditation) and 27 had neither accreditation nor certification. Overall percentage scores for 229 criteria of quality and safety were 66.9, 60.0 and 51.2, respectively. Analysis confirmed statistically significant differences comparing mean scores by the type of external assessment (accreditation, certification or neither); however, it did not substantially differentiate between accreditation and certification only. Some of these associations with external assessments were confounded by the country in which the sample hospitals were located. CONCLUSIONS: It appears that quality and safety structures and procedures are more evident in hospitals with either the type of external assessment and suggest that some differences exist between accredited versus certified hospitals. Interpretation of these results, however, is limited by the sample size and confounded by variations in the application of accreditation and certification within and between countries.