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Weedy forms of cultivated sunflower (Helianthus annuus) are invasive species widely distributed in several regions of the world and are commonly controlled by applying acetohydroxyacid synthase (AHAS)-inhibiting herbicides, such as imidazolinones (IMIs) or sulfonylurea (SUs). The widespread adoption of herbicide-resistant crops has exposed the weedy population to the high risk of crop-to-weedy gene flow. The aim of this study was to check and quantify the gene flow from IMI- and SU-resistant sunflower hybrids to weedy sunflower populations. Field experiments were conducted in 2008 at two sites in Serbia to evaluate the relationship of distance between the crop and the weedy sunflower and its impact on the percentage of gene flow. The weedy sunflower progenies were evaluated through herbicide resistance and SSR marker study. Hybridization with IMI-resistant hybrids was not confirmed. Conversly, SU-resistance trials and SSR marker studies partially confirmed the transfer of resistance within the weedy population.