This paper presents evaluations of the impact of six advanced fuel cycles, ranging from the present "once-through" fuel cycle in light water reactors to a gas-cooled fast reactor with fully recycling of all actinides, on geological disposal in a clay formation. Both the dimensions and the radiological consequences of a geological repository for the disposal of high-level radioactive waste (HLW) and spent fuel are estimated. After a 50-yr cooling time, the thermal output of the HLW arising from advanced fuel cycles is significantly lower than that of spent fuel. This allows the dimensions of the geological repository to be reduced. The impact of advanced fuel cycles on the radiological consequences in the case of the expected evolution scenario is rather limited. The maximum dose, which is expected to occur a few tens of thousands of years after the disposal of the waste, is essentially due to fission products, and their amount is approximately proportional to the heat generated by nuclear fission. An important contributor to the total dose is 129I; the amount of 129I going into a repository strongly depends on the fraction of spent fuel that is reprocessed. By considering the evolution of the radiotoxicity of the waste, it can be expected that the radiological consequences of human intrusions into a repository will be significantly lower in the case of waste arising from advanced fuel cycles.