Objective. To determine any additional value in the evaluation of safety levels by adding an appended oncology module to the Institute for Healthcare Improvement's Global Trigger Tool (GTT). Design. Comparison of two independent retrospective chart reviews: one review team using the general GTT method and one using the general GTTmethod plus the appended oncology module on the same inpatient charts. Setting. The Department of Clinical Oncology at a Danish University Hospital (1000 beds). Participants. All inpatients admitted to the hospital in 2010, n = 3692, biweekly sample of 10 admission charts resulting in a double review of 240 charts. Main Outcome Measures. Total number of identified adverse events (AEs), distribution of identified AEs in the harm categories of the National Coordinating Council for Medication Error Reporting and Prevention (NCC MERP), AEs per 100 admissions and AEs per 1000 admission days. Results. No significant (95% confidence interval) difference was found between review teams using the general GTT versus the general GTT plus the appended oncology module on the total number of identified AEs, AEs per 100 admissions, AEs per 1000 admission days or in the overall distribution of identified AEs in the five NCC MERP harm categories. Conclusions. The study showed that adding the appended oncology module to the GTT did not increase its value regarding the evaluation of safety levels. This finding could be due to the measurement error of the GTT. Further studies evaluating the measurement properties and the specific additional modules to the general GTT are needed.