Objective To determine the change in adverse event (AE) rates and preventable AE rates over time, identify certain patient risk groups and discuss factors influencing the outcome.

Design Longitudinal retrospective patient record review study.

Setting and participants A random sample of 21 hospitals in The Netherlands in 2004, and 20 hospitals in 2008. In each hospital, 400 patient admissions were included in 2004, and 200 in 2008.

Main outcome measures AEs and preventable AEs.

Results Multilevel analyses of 11 883 patient records (7.887 in 2004, 3.996 in 2008) showed that the rate of patients experiencing an AE increased from 4.1% (95% CI 3.3% to 5.1%) in 2004 to 6.2% (95% CI 5.0% to 7.6%) in 2008. The preventable AE rate remained relatively stable at 1.8% (95% CI 1.3% to 2.4%) in 2004 and 1.6% (95% CI 1.2% to 2.3%) in 2008. The risk of experiencing a preventable AE was increasingly higher for patients admitted to a surgical unit (OR 1.54 (95% CI 1.10 to 2.16) in 2004 and 3.32 (95% CI 2.17 to 5.07)) in 2008. More than 50% of all AEs were related to surgery. Indications were found that differences in the risk of experiencing a preventable AE between hospital departments were larger in 2008 than in 2004, while differences between hospitals themselves were smaller.

Conclusions Patient harm related to healthcare is a persistent problem that is hard to influence. Measuring AEs over time stresses the continuing urgency, and also identifies possible areas for improvement.