In spite of the growing interest in nursing ethics, few studies have focused on ethical dilemmas experienced by nurses working with clinical studies as 'research nurses'. The aim of the present study was to describe and explore ethical dilemmas that Swedish research nurses experience in their day-to-day work. In a qualitative study a purposeful sample of six research nurses from five wards of differing disciplines in four Swedish hospitals was interviewed. The analysis displayed several examples of ethical dilemmas, primarily tensions between the nurses' obligations to the study and to the patients involved. A guiding moral principle for the nurses was patient-centeredness, where the interest of research must not override the interest of the patient. In situations where tensions between research and patient interests occurred, and doctors and nurses disagreed upon the judgement, the nurses sometimes chose to follow the doctors' advice, and thus acted against their own moral judgment. Such situations seemed to create feelings of moral distress among the nurses. They described their profession as being 'invisible' and as lacking opportunities for ethical competence building. The conclusion is that research nurses frequently experience severe and difficult ethical dilemmas in their daily work. They need to be acknowledged as a particular profession in the health care organisation and encouraged to develop their specific ethical competence.