Background and Aims: Grafting is used in viticulture worldwide. Rootstocks are known to alter scion development or 'confer vigour' to varying degrees. This work examines scion/rootstock interactions in young grafted grapevines. Its aim was to determine the effects of scion and rootstock genotypes on biomass allocation within the plant. Methods and Results: Five months after grafting, biomass allocation between the root and the shoot was measured for all the scion/rootstock combinations made between three Vitis genotypes. The scion genotype explained the highest percentage of the non-random variance for biomass allocation within the plant, including biomass allocation to the roots, i.e. it conferred differences in root vigour. In addition, we developed a double-grafted system, in which a single scion was grafted onto two rootstocks, to analyse further scion (or carbon source) effects on root development. Conclusions: This work provides evidence of conferred root vigour by the scion which does not appear to be related to carbon supply from the shoot. The genotypes studied display varying levels of plasticity in their response to different grafting partners. Significance of the Study: This work presents detailed analysis of biomass allocation within young grafted vines. In young grafted grapevines, the scion genotype has a major effect on most parameters of development, especially in the root. This aspect of rootstock/scion interactions should be taken into account when selecting rootstocks. A double-grafting system was developed for future research on signalling pathways and exchange mechanisms between scion and rootstock in grafted grapevine.