In 1979, a study indicated that childhood cancers might be linked with extremely low frequency (ELF) magnetic field [1]. Leaning on numerous epidemiological studies, the International Agency for Research on Cancer classified in 2001 ELF MF as "possibly carcinogenic to human". These conclusions were based on a statistical association found in some epidemiological studies, unconfirmed by experimental results, between childhood leukaemia risk and a mean exposure over 24h higher than 0.4μT. One remaining -and critical- question about these epidemiological results is the exposure assessment. Another relevant question is to know, in the overall exposure of people, the relative contribution of all the possible field sources in the daily environment. Starting from a previous study (50 Hz magnetic field levels in houses, half of them close to power lines [2]), the French Ministry of Health initiated in 2006 a large study of the individual exposure of a randomly selected sample of the French population. The total exposure database will contain 1000 measurements for children and 1000 measurements for adults. The analysis of results is still ongoing, and should be published in 2010. This paper presents intermediate results, based on the data already analysed.