To estimate the introgressive hybridisation propensity (IHP) between transgenic oilseed rape and certain of its crosscompatible wild /weedy relatives at the landscape level, a conceptual approach was developed. A questionnaire was established enclosing the successive steps to successfully achieve introgressive hybridisation. Each step was described and scored, resulting in an IHP value for each cross-compatible wild /weedy relative. This approach revealed that in Flanders (Belgium) Brassica rapa has the highest IHP value, followed by Hirschfeldia incana, Diplotaxis tenuifolia, Raphanus raphanistrum and Sinapis arvensis. Using these values, monitoring priorities can be defined within the pool of cross-compatible wild /weedy oilseed rape relatives. It is discussed how the numerical quantification may serve as a valuable tool in case-specific monitoring and general surveillance of transgenic oilseed rape. -® 2007 Birkhaueser