The purpose of the present paper is to provide guidelines about the implementation of epidemiological studies on populations exposed to particularly high levels of 50 Hz magnetic fields, indicatively units of microT. The available evidence, in fact, points to a consistent increase of the occurrence of neoplastic and neurodegenerative disease and adverse reproductive outcomes associated to these levels of exposure. An approach based on the detection of high risk groups is proposed. In this frame contrastability between study subjects and suitable reference populations is the core issue. Some procedures aimed at selecting individuals and populations to be studied are discussed, in order to support local environmental and health authorities in setting priorities for studies and interventions. Finally, the implications of these studies in terms of scientific knowledge and environmental reclamation are examined.