This paper discusses the importance of protecting the privacy of patient data kept in an Electronic Health Record (EHR) in the case, where it leaves the control- and protection-sphere of the health care realm for secondary uses such as clinical or epidemiological research projects, health care research, assessment of treatment quality or economic assessments. The paper focuses on multi-centric studies, where various data sources are linked together using Grid technologies. It introduces a pseudonymisation system which enables a multi-centric universal pseudonymisation, meaning that a patient's identity will result in the same pseudonym, regardless of which participating study center the patient data is collected.