The possibility of gene flow between cultivated and wild species of different crops was assessed, focussing on sunflower and its wild relatives or cultivated forms (annual Helianthus annuus, H. argophyllus, H. bolanderi, H. petiolaris, and perennial H. rigidus, H. tuberosus). Data were compiled from the literature and an attempt was made at determining the efficiency of possible crosses between annual and perennial species. The risk of a cultivated allele escaping from sunflower to uncontrolled forms (volunteers, H. annuus and H. tuberosus feral forms) was addressed, the fate of such alleles in the wild also being examined. An attempt was made to define some of the pressures likely to limit the spread of wild forms in nature, or that could prevent or impede the spread and increase the frequency of alleles from cultivated plants. Risks of spread of transgenes from sunflower varieties in Europe where feral forms are present but no natural Helianthus species were assessed. The emergence of transgenic or biologically engineered varieties of sunflower demands that attention be focussed on this field.