Prospective population based cohort studies in the field of psychiatry are crucial in order to study the etiology of complex and multi-factorial diseases such as psychiatric disorders. From the ethical point of view, however, the collection of longitudinal data poses several challenges to psychiatrists, mainly related to the processing of personal data. Respecting and protecting the confidentiality of data and the privacy of individuals regarding the information that they have given as participants in a research project is a corner stone of complying with accepted research standards. However, in longitudinal studies establishing and maintaining privacy is often challenging due to the necessity of repeated contact with participants. A novel internet-based solution is introduced here, maintaining privacy at the same time as it assures linkage of data to individual participants in a repeated measures design. Using the Anonymous Repeated Measurements via Email procedure (ARME), two separate one-way communication systems are established through ad hoc e-mail accounts and a secure study website. Strengths and limitations of the approach are discussed.