Australian sweet lupins (Lupinus angustifolius L.) and their naturalised wild progenitor occur widely throughout the agricultural zone of Western Australia. Before unrestricted release of transgenic lupins is allowed, an assessment is needed of the likely level of gene flow between the wild and cultivated lupins. Three sets of data were collected to evaluate the likelihood of outcrossing and gene flow. These were the level of outcrossing between adjacent lupin crops, the spatial distribution of crops and wild lupins, and the relative flowering times of the crops and wild lupins. The level of outcrossing within the first 1.5 m of adjacent crops, assessed over 1.56 million plants, was 1 cross in 3600 plants. No crossing was detected at distances greater than 2.25 m (outcrossing rate less than 1 in 148 000). The distribution of 216 crops and 237 wild populations in Western Australia rarely overlapped. Only 5 wild populations were detected in the area where crops were found. No crops occurred in the areas where the remaining wild lupins (232 populations) were found. The crops were all early flowering, whereas the 3 selected wild populations, representing 3 different (medium, high, and very high rainfall) climatic zones, all flowered later. We conclude that the likelihood of gene flow from a transgenic lupin crop to wild lupin populations is extremely low. This is consistent with the fact that reverse gene flow from wild lupins carrying dominant genes for blue flowers and bitter seeds has never been found in farmers’ fields, despite 35 years of lupin cropping where areas of over 1 000 000 ha per year have been grown in Western Australia.