Is returning to a conventional crop possible after growing a genetically modified (GM) variety?  Oilseed rape volunteers in oilseed rape crops can cause seed admixture in the harvest.  Farmers who desire to go back to conventional varieties after growing genetically modified varieties must comply with the 0.9% threshold of GM material in the harvest if they are to declare it as non-GM for food and feed use in Europe.  The farm-scale GM platforms set up in France in 1995 were used to estimate the amount of GM seeds in the harvest of a conventional variety grown 3-8 years later in the same field.  We show that over time the rate of GM seeds admixture in the harvest largely exceeded the European threshold in 6 out of 18 cases.  In one case, the GM level was as high as 18%.  However, it varied considerably according to the varieties used.  These findings are discussed in the light of studies about seed shedding, seed dormancy and survival in the soil which are suggested to be the main source of large differences among GM varieties, which ultimately determines the frequency of volunteer plants growing amongst and harvested in admixture with the sown crop.  These findings are equally discussed in light of differences of growth habit and potential competitiveness between conventional varieties responsible for different seed output by volunteers.  To help cope with coexistence over time, we discuss appropriate uses of varieties and crop management in order to prevent seed admixture.