U-series were investigated in groundwaters from two water bodies of the Eocene sands aquifer, southwest France. The 234U/238U activity ratios are always larger than the equilibrium value, some waters being highly enriched in 234U (as large as 13.5). In the southern water body, 234U/238U ratios correlate with 14C ages (with one exception). Such U isotopic signatures may allow the evaluation of the residence time of groundwaters within their aquifer using an open-system model based on U activity ratios and concentrations. It is suggested that the ‘oldest’ groundwaters from the southern water body are no older than 38 ky, whereas the maximum ‘age’ of groundwaters from the northern water body, though overall of the same order of magnitude, could be slightly younger (25 ky).