Psychosocial reluctance is a major obstacle to the success of mass screening. In France, several opinion polls demonstrated that the fear of cancer as a disease, the fear of a positive result and the fear of the possible therapeutic consequences were the major determinants opposed to screening despite information often known to the public. The psychological obstacles appear less important in the US population, except for minorities. Age seems to be a discriminating factor, indeed after 50 years, reluctance to screening is increased probably due to a generation effect influencing the meaning of the disease and the interest of screening. Site specific constraints also exist, in colorectal cancer, 30% of the people feel as an embarrassment the manipulation of the stools. If false positives are responsible for a supplementary anxiety, it doesn't last long and it doesn't influence the observation of future campaign. These psychosocial obstacles as a whole justify the need of converging information that must take into account cultural problems. The favourable influence of the general practitioner explains his essential role for the success of mass screening.