Research objective: Reliable and unambiguously defined performance indicators are fundamental to objective and comparable measurements of hospitals’ quality of care. In two separate case studies (intensive care and breast cancer care), we investigated if differences in definition interpretation of performance indicators affected the indicator scores. Design : Information about possible definition interpretations was obtained by a short telephone survey and a Web survey. We quantified the interpretation differences using a patient-level dataset from a national clinical registry (Case I) and a hospital’s local database (Case II). In Case II, there was additional textual information available about the patients’ status, which was reviewed to get more insight into the origin of the differences. Participants : For Case I, we investigated 15 596 admissions of 33 intensive care units in 2009. Case II consisted of 144 admitted patients with a breast tumour surgically treated in one hospital in 2009. Results: In both cases, hospitals reported different interpretations of the indicators, which lead to significant differences in the indicator values. Case II revealed that these differences could be explained by patient-related factors such as severe comorbidity and patients’ individual preference in surgery date. Conclusions : With this article, we hope to increase the awareness on pitfalls regarding the indicator definitions and the quality of the underlying data. To enable objective and comparable measurements of hospitals’ quality of care, organizations that request performance information should formalize the indicators they use, including standardization of all data elements of which the indicator is composed (procedures, diagnoses).